Deduplicate macro_rules! from module_exports when documenting them
Deduplicate macro_rules! from module_exports when documenting them

This can append if within the same module a `#[macro_export] macro_rules!`
is declared but also a reexport of itself producing two export of the same
macro in the same module. In that case we only want to document it once.

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust