[RFC/WIP] Use fixed-width integers

I'm gonna have some shots at fixing `-Wconversion` for the old files, but I before we need to use fixed-width integers on most places(this would be needed anyway for consistency between windows/unixes)

Here's the script used so far:

```sh
#!/bin/bash
find src -name '*.h' -or -name '*.c' -print0 | xargs -0 sed -i\
	-e 's/\blong_u\b/uint64_t/g' \
	-e 's/\bunsigned\s\+long\s\+long\s\+int\b/uint64_t/g' \
	-e 's/\bunsigned\s\+long\s\+long\b/uint64_t/g' \
	-e 's/\bunsigned\s\+long\s\+int\b/uint64_t/g' \
	-e 's/\bunsigned\s\+long\b/uint64_t/g' \
	-e 's/\blong\s\+long\s\+int\b/int64_t/g' \
	-e 's/\blong\s\+long\b/int64_t/g' \
	-e 's/\blong\s\+int\b/int64_t/g' \
	-e 's/\bunsigned\s\+short\b/uint16_t/g' \
	-e 's/\bunsigned\s\+int\b/uint32_t/g' \

git checkout src/nvim/lib/k{list,hash}.h
```

Does anyone has an idea of how I can tell sed skip comments?