Add support for implicit type conversions to ISLE.
This feature allows the DSL user to register to the compiler that a
particular term (used as a constructor or extractor) converts from one
type to another. The compiler will then *automatically* insert this term
whenever a type mismatch involving that specific pair of types occurs.
This significantly cleans up many uses of the ISLE DSL. For example,
when defining the compiler backends, we often have newtypes like `Gpr`
around `Reg` (signifying a particular type of register); we can define
a conversion from Gpr to Reg automatically.
Conversions can also have side-effects, as long as these side-effects
are idempotent. For example, `put_value_in_reg` in a compiler backend
has the effect of marking the value as used, causing codegen to produce
it, and assigns a register to the value; but multiple invocations of
this will return the same register for the same value. Thus it is safe
to use it as an implicit conversion that may be invoked multiple times.
This is documented in the ISLE-Cranelift integration document.
This PR also adds some testing infrastructure to the ISLE compiler,
checking that "pass" tests pass through the DSL compiler, "fail" tests
do not, and "link" tests are able to generate code and link that code
with corresponding Rust code.
