Prepare for v1.6

It's time again for the next release, which will be v1.6. It should be released this weekend. Things on the list:

~~[ ] addition of mpytool.py to inspect .mpy files~~
[ ] ability to use mpytool.py to freeze .mpy files so precompiled Python code can be executed from flash (basically this is "true" frozen modules, although I think it's good to keep the existing "frozen script" frozen modules as well)
merge stmhal: USB CDC speed improvements, TIM3 release #1713 (stmhal CDC improvements)
merge lib/utils/printf: Fix printf on release builds #1799 (unix printf fix)
unix port segfault unix: the GC does not scan all relevant parts of the stack #1787
stmhal I2C bus errors with DMA: stmhal: tests/pyb/halerror.py leaves i2c in a weird state. #1683 unable to recover from I2C bus error #1765

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