Optimise BasicMode::clear RAM usage and optimize size of Ssd1306::char_to_bitmap

[alexey-medvedchikov]

Hi! Thank you for helping out with SSD1306 development! Please:

• [ ] Check that you've added documentation to any new methods
• [X] Rebase from master if you're not already up to date
• [ ] Add or modify an example if there are changes to the public API
• [X] Add a CHANGELOG.md entry in the Unreleased section under the appropriate heading (Added, Fixed, Changed, etc)
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PR description

This PR fixes issues with small AVRs:

• BasicMode doesn't work on MCUs with less than ~1.5Kb of RAM because there's a stack array of 1024 bytes allocated to zero-out pages. Instead, we can zero-out in small pieces.
• Refactor glyph search to use lookup table instead of one big match. Saves around 3.5Kb of compiled code on AVR ATMega-s. Didn't test on other architectures, but pretty sure it will save some size too.

AVR size difference

Before:

text    data     bss     dec     hex filename
   0    8446       0    8446    20fe object.hex

After:

text    data     bss     dec     hex filename
   0    4988       0    4988    137c object.hex

STM32 (thumbv7em) size difference

Before:

verified 7212 bytes

After:

verified 6392 bytes

[alexey-medvedchikov]

@jamwaffles No problemo!

Although I saved some space, I still can't squeeze my program into atmega168 - bme680 driver is a resource hog (making a small weather station with a screen). So I'll try to switch to a STM32 MCU, I can test this change there too later this week. I also have ESP8266 at hand, but I've never used it with Rust, so not sure if I can quickly check it.

[alexey-medvedchikov]

LOL, I've just noticed that all the first and last bytes of each glyph in the font are zeros, so I just made the data array smaller by another 500 bytes. It is a bit clunky, but the font is hardcoded (and quite nice), so I don't think it is a problem. -3.5Kb total on atmega.

[alexey-medvedchikov]

Tested on NUCLEO-F446ZE and ATMega168 with 128x64 and 128x32, all work fine.

[alexey-medvedchikov]

Got rid of extra-sized arrays in Command::send, cuts 80 bytes on Cortex-M4 and 260 bytes on ATMega168.

[alexey-medvedchikov]

Thanks for the review. I accepted all your suggestions, a newbie in Rust, so make simple mistakes. Also, the merge conflict should be resolved now.

[alexey-medvedchikov]

@jamwaffles checked and committed your changes. Thanks for spending so much time on this one!

[jamwaffles]

This is released in v0.8.3. cargo update should pull in the changes.

[bugadani]

FYI I'm firmly opposed to https://github.com/jamwaffles/ssd1306/pull/195/commits/e309b41027e4219a17ba28200034eddcff25da77 as it means I'll have to duplicate the whole command table for the async interface.