use less unstable dependencies

[japaric]

This PR and the ones at the bottom reduce the number of unstable features needed for Cortex-M development to a single one: lang = "panic_fmt", which already has a path towards stabilization and which we hope to get on stable by 1.28.

Check out the temporary documentation (we still need more docs) to try out this preview.

We would love your input on these unresolved questions

This PR depends on:

• japaric/cortex-m-rt#69
• japaric/cortex-m#88
• japaric/panic-semihosting#2
• japaric/svd2rust#203
• japaric/stm32f103xx#24

[japaric]

Something I noticed while working on this is that LLD (both HEAD and rustc's) will mislink FFI calls into external assembly files. LLD will insert a BLX instruction that calls the foreign function instead of a BL instruction. The BLX instruction produces a hard fault at runtime.

cc @jcsoo

[japaric]

bors r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push