Half-fix type caching for unsigned types (- don't) + added TODO
const_fold_val returns the value inline
Don't load a constant to a register if it's possible to inline

i.e. if the operator is commutative, we can inline a constant in the
instruction.
Don't generate shifting-zero or zero-width shifts
Cast to long-long for out_op's binary-& optimisation check
bitfield example and tests
Bitfield structure packing
Added ICE for zero-width bitfields
Bitfield framework + bitfield loading
impl_op(): force constant to register if we have two constants
bitfield storing (via read + masking)
Added warning note about constant truncation
Added bitfield TODO