All of the instruction format an opcode methods are emitted as an
InstBuilder trait instead of adding them to the Bulder struct directly.
The methods only make use of the InstBuilderBase methods to create new
instructions.
This makes it possible to reuse the InstBuilder trait for different ways
of inserting instructions.
RISC-V does not have a flags register, and thus no add-with-carry
instructions. Neither does MIPS.
Add expansions of these instructions in terms of iadd and icmp.
The 'lib/cretonne' directory will be the new root of a stand-alone
cretonne crate containg both Python and Rust sources.
This is in preparation for publishing crates on crates.io.
