Move bitfields into the IR

This commit moves bitfields and the computation of their allocation units into
the IR. They were previously computed on-the-fly during code generation. In
addition to breaking up and compartmentalizaing a portion of the gargantuan
`CodeGeneration` implementation for `CompInfo`, this paves the way for adding
const-fn bitfield unit constructors.

Author: fitzgen

Cargo.lock

@@ -46,6 +46,7 @@ cexpr = "0.2"
 cfg-if = "0.1.0"
 clang-sys = { version = "0.17.0", features = ["runtime", "clang_3_9"] }
 lazy_static = "0.2.1"
+peeking_take_while = "0.1.2"
 syntex_syntax = "0.58"
 regex = "0.2"
 # This kinda sucks: https://github.com/rust-lang/cargo/issues/1982