Add regex and CAN, improve UART, and more.

py core:
- add basic regex support using re1.5 library
- replace zlibd with uzlib
- compiler returns proper exception on SyntaxError
- proper context saving for eval/exec
- use mp_[u]int_t in most places instead of [u]int
- better ARM support in native emitter
- autodetect machine endianness
- better big endian support
- memory errors from locked gc can be reported
- more robust lexer when out of memory
- rename modules to begin with u-, and add module weak links
- more native binops in viper emitter
- native pointers with load+store in viper
- fix GC realloc so that chunks are shrunk correctly
- compiler optimisations for constant if/while conds
- add kwargs for builtin open() and FileIO
- enable unicode on windows

stmhal port:
- add basic CAN bus support
- overhaul UART class to use read/write
- add NRF24L01 driver (in pure Python)
- pyb.freq can change SYSCLK
- add pyb.stop and pyb.standby
- can set Timer freq using float
- Timer supports complementary channel and deadtime
- add retry for SD card init