[v1.0.0] Support signed integers with arbitrary bits (e.g. int24)

[hit9]

Signed integers supports:

• [x] C (lib version)
• [x] C (without lib version, optimization mode)
• [x] Go (lib version)
• [x] Go (without lib version, optimization mode)
• [x] Python

Review:

• [x] Review

Misc stuffs:

• [x] (compiler) Reformat imports
• [x] unittests
• [x] docs
• [ ] package (binary) after merge..

Target version:

compiler: v0.4.6 bitprotolib (python): v0.1.2