auto merge of #5264 : brson/rust/cross7, r=brson

Follow up to #5242. All credit to @yichoi.
Permit casting region pointers to unsafe ones.
Inline constants across crates.
Allow casting integers to unsafe pointers in constant expressions.
auto merge of #5227 : jdm/rust/newconst, r=pcwalton

All comments from previous pull requests addressed.
core: Add rt mod and add the new scheduler code