Experience with the `define_function` API has shown that returning
borrowed slices of `TrapSite` is not ideal: the returned slice
represents a borrow on the entire `Module`, which makes calling back
into methods taking `&mut self` a bit tricky.
To eliminate the problem, let's require the callers of `define_function`
to provide `TrapSink` instances. This style of API enables them to
control when and how traps are collected, and makes the `object` and
`faerie` backends simpler/more efficient by not having to worry about
trap collection.
* move trap site definitions into cranelift-module
`cranelift-faerie` and `cranelift-object` already have identical
definitions of structures to represent trap sites. We might as well
merge them ahead of work to define functions via a raw slice of bytes
with associated traps, which will need some kind of common structure for
representing traps anyway.
* cranelift-module: add `define_function_bytes` interface
This interface is useful when the client needs to precisely specify the
ordering of bytes in a particular function.
* add comment about saving files for `perf`
