Merge #1290 #1291 #1294

1290: Reformat copyright header in rust-parse-impl.h r=philberty a=philberty



1291: Fix bad impl item overlap check r=philberty a=philberty

This issue here was the equality checks for reference
and pointer types did not check the mutability.
    
Fixes #1289

1294: Add new mappings for items within a module r=philberty a=philberty

This patch adds two new interfaces to the mappings class.

  - Lookup the child items within a module
  - Lookup the parent module id from the child item id

Each of these API's are going to be required so that we can resolve two new
types of path segment:

  super::foo::bar

Where we need to be able to lookup the super module node id from the
current scope node id. To then lookup the children within that scope. The
other path is simpler such as: crate::foo::bar. Where we lookup the items
within the current Crate NodeId and follow on as normal.

Addresses #1227

Co-authored-by: Philip Herron <philip.herron@embecosm.com>