Use okio

[mvysny]

okio is a rich multi-platform library which introduces lots of support for working with files and streams, buffered streams, etc. Add implementation("com.squareup.okio:okio:3.2.0") to build.gradle.kts to add the library.

Problems:

• The default unix impl of Source/Sink/UnixFileHandle accesses the file via CPointer<FILE>. We need int32_t fd to work with serial port's tcsetattr; yet we might use fileno
• FileSource.file is private; can't access that via reflection :-D
• Source/Sink are unidirectional while we need bidi; that can be worked around by following the example of Socket at https://square.github.io/okio/recipes/
• I can implement a fd-based Source/Sink and bolt support for serial port on top of that; we could then still use the rich api goodness of Path and Buffered from okio.
• BLOCKER: okio doesn't compile to arm yet.

[mvysny]

Blocked by https://github.com/square/okio/issues/1171