Measure test coverage of rust code

[mmilata]

Unit tests - easy but not very useful. There seem to be two main tools:

• cargo-llvm-cov - LLVM instrumentation based, seems to work out of the box
• cargo-tarpaulin - either LLVM instrumentation or ptrace on Linux

Emulator - device tests etc.

• gcov compatible / -Z coverage - no longer supported, it seems
• LLVM instrumentation: currently trezor_lib compiles successfully with -C instrument-coverage but not .profraw file is written
  • perhaps we need to initialize the profiling runtime manually?
  • not sure if it even works with crate type staticlib or when using GCC as the final linker