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: Remove uses of DVec in io/repr
std: Remove uses of DVec
rustc: Remove uses of DVec
syntax: Remove uses of DVec
rustc change