Merge #1739

1739: Initial type bounds checking for all type checks r=philberty a=philberty

This patchset is my branch for general improvements to the type-system and
for improved type bounds checking in preparation for opaque types.

Fixes #1773 #1786

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

diff --cc gcc/rust/backend/rust-compile-expr.cc
Simple merge