Removed a few macro-expanding-to-module workarounds

Also documented a few issues
Removed useless cmp::{min, max} reexports from the integer modules
Removed module macro workaround for signed and unsigned integers
auto merge of #10719 : Kimundi/rust/switch_to_multi_item_macros, r=alexcrichton

- Removed module reexport workaround for the integer module macros
- Removed legacy reexports of `cmp::{min, max}` in the integer module macros
- Combined a few macros in `vec` into one
- Documented a few issues