Generate nice doc comments for the Opcode enum variants that 'cargo doc'
will pick up.
Include a `Display` trait implementation that prints the lower
snake-case version of the opcode name.
Add new immediate types for floating point and vector immediates.
Use new immediates to define the constant value instructions in meta.
Split the fconst instruction into two: f32const and f64const. This prevents
confusion about the interpretation of 64 immediate bits when generating an f32
constant.
Add an immvector ImmediateType. This immediate type is variable length, and
provides all the bits of a SIMD vector directly.
The Cretonne meta language is used to describe Cretonne instructions, both the
target independent ones in the base instruction set and real target
instructions.
Start by providing type definitions matching langref, and begin the meta
language reference using autodoc to pull in the PYthon definitions.
