[LINTER] Add a linter to replace unsigned char with uint8_t

Summary:
This is a recurrent issue in reviews, this diffs provides an autofix
linter for this.
It is named afted stdint so that other conversion can be lated added if
needed.

Test Plan:

arc lint --everything # No warning

Edit src/init.cpp to add the line:

unsigned char foo;

anywhere in the file, then run:

arc lint -- src/init.cpp

The linter should catch the line and suggest a fix.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3592