Fixed openNextFile() speed in case of large directories.

[dikdom]

In case of large directories the SD::openNextFile() iterates through the files and opens them one by one. It calls SdFile::open() by name, so the open() method also iterates through the directory until it finds the selected file. Instead let's keed track the directory index (position within the directory) and open the file by this index. This speeds up the iteration on large directories dramatically.

Also a typo is fixed in the README.adoc. From the author, you shall remove my name.

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[github-actions[bot]]

Memory usage change @ dc2ea3e36b3ed14ca37e19327996b3d1abc5e4c3

Board	flash	%	RAM for global variables	%
arduino:avr:leonardo	:small_red_triangle: 0 - +182	0.0 - +0.63	0 - 0	0.0 - 0.0
arduino:avr:mega	:small_red_triangle: 0 - +182	0.0 - +0.07	0 - 0	0.0 - 0.0
arduino:avr:nano	:small_red_triangle: 0 - +182	0.0 - +0.59	0 - 0	0.0 - 0.0
arduino:mbed_nano:nano33ble	:small_red_triangle: 0 - +64	0.0 - +0.01	0 - 0	0.0 - 0.0
arduino:mbed_nano:nanorp2040connect	:small_red_triangle: 0 - +88	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:mbed_portenta:envie_m4	:small_red_triangle: 0 - +64	0.0 - +0.01	0 - 0	0.0 - 0.0
arduino:mbed_portenta:envie_m7	:small_red_triangle: 0 - +64	0.0 - +0.01	0 - 0	0.0 - 0.0
arduino:megaavr:nona4809	:small_red_triangle: 0 - +182	0.0 - +0.37	0 - 0	0.0 - 0.0
arduino:sam:arduino_due_x_dbg	:small_red_triangle: 0 - +80	0.0 - +0.02	N/A	N/A
arduino:samd:mkrzero	:small_red_triangle: 0 - +92	0.0 - +0.04	0 - 0	0.0 - 0.0

Click for full report table

Board|examples/CardInfo
flash|%|examples/CardInfo
RAM for global variables|%|examples/Datalogger
flash|%|examples/Datalogger
RAM for global variables|%|examples/DumpFile
flash|%|examples/DumpFile
RAM for global variables|%|examples/Files
flash|%|examples/Files
RAM for global variables|%|examples/NonBlockingWrite
flash|%|examples/NonBlockingWrite
RAM for global variables|%|examples/ReadWrite
flash|%|examples/ReadWrite
RAM for global variables|%|examples/listfiles
flash|%|examples/listfiles
RAM for global variables|% -|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|- arduino:avr:leonardo|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|182|0.63|0|0.0 arduino:avr:mega|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|182|0.07|0|0.0 arduino:avr:nano|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|182|0.59|0|0.0 arduino:mbed_nano:nano33ble|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|64|0.01|0|0.0 arduino:mbed_nano:nanorp2040connect|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|88|0.0|0|0.0 arduino:mbed_portenta:envie_m4|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|64|0.01|0|0.0 arduino:mbed_portenta:envie_m7|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|64|0.01|0|0.0 arduino:megaavr:nona4809|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|182|0.37|0|0.0 arduino:sam:arduino_due_x_dbg|0|0.0|N/A|N/A|0|0.0|N/A|N/A|0|0.0|N/A|N/A|0|0.0|N/A|N/A|0|0.0|N/A|N/A|0|0.0|N/A|N/A|80|0.02|N/A|N/A arduino:samd:mkrzero|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|0|0.0|92|0.04|0|0.0

Click for full report CSV

``` Board,examples/CardInfo
flash,%,examples/CardInfo
RAM for global variables,%,examples/Datalogger
flash,%,examples/Datalogger
RAM for global variables,%,examples/DumpFile
flash,%,examples/DumpFile
RAM for global variables,%,examples/Files
flash,%,examples/Files
RAM for global variables,%,examples/NonBlockingWrite
flash,%,examples/NonBlockingWrite
RAM for global variables,%,examples/ReadWrite
flash,%,examples/ReadWrite
RAM for global variables,%,examples/listfiles
flash,%,examples/listfiles
RAM for global variables,% arduino:avr:leonardo,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,182,0.63,0,0.0 arduino:avr:mega,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,182,0.07,0,0.0 arduino:avr:nano,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,182,0.59,0,0.0 arduino:mbed_nano:nano33ble,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,64,0.01,0,0.0 arduino:mbed_nano:nanorp2040connect,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,88,0.0,0,0.0 arduino:mbed_portenta:envie_m4,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,64,0.01,0,0.0 arduino:mbed_portenta:envie_m7,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,64,0.01,0,0.0 arduino:megaavr:nona4809,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,182,0.37,0,0.0 arduino:sam:arduino_due_x_dbg,0,0.0,N/A,N/A,0,0.0,N/A,N/A,0,0.0,N/A,N/A,0,0.0,N/A,N/A,0,0.0,N/A,N/A,0,0.0,N/A,N/A,80,0.02,N/A,N/A arduino:samd:mkrzero,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,92,0.04,0,0.0 ```

[dikdom]

I have agreed the CLA. I am a GitHub user.