Merge #1385 #1386 #1387 #1388

1385: Add missing include for intellisense r=philberty a=philberty



1386: Fix undefined behaviour using .get on unique_ptr r=philberty a=philberty

As the move semantics for AST and HIR crates are unsafe on older compilers
we are working around this by storing the pointer into the mappings class
this was using the get method to store the pointer. The issue for loading
extern crates the unique_ptr goes out of scope and this the deletion code
resulting in undefined behaviour. This changes it to call release to take
full ownership of the pointer as we expect.


1387: External Items with Rust ABI need to mangle the full path r=philberty a=philberty

When compiling external rust abi item requires the fully qualified
canonical path to be mangled in order to link correctly.


1388: Add new interfaces to mappings class r=philberty a=philberty

In order to load and resolve extern crates we need to support detection
of node_id is a crate in order to support paths outside of the current
crate.


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