Define data types for the level 1 and level 2 hash tables. These data types are
generic over the offset integer type so they can be twice as compact for
typically small ISAs.
Use these new types when generating encoding hash tables.
Emit both level 1 and level 2 hash tables.
Define generic functions that perform lookups in the encoding tables.
Implement the TargetIsa::encode() method for RISC-V using these building
blocks.
Add an isa::lookup() function which serves as a target registry for creating
Box<TargetIsa> trait objects.
An isa::Builder makes it possible to confugure the trait object before it is
created.
