Moved Rust bindings to Rust 2018 edition

Only include idle task stack usage check if DIAG_ENABLE

Implemented task stack size diagnostic messages

Removed diag task switch message

Initial implementation of stack painting.

Renamed the Mouros Rust binding crate

Added getter function for os_tick_count

Added microsecond active wait function

Changed the Mailbox channels to resemble Rust channels

Removed unnecessary mutable specifier

Added SPSC Rust channels

Implemented a critical section lock structure

Added initial version of a Rust pool allocator

Updated libopencm3 version

Use alternate ARM GCC in Travis builds