This commit is a slight refactoring of the `Module` trait and backend in
`cranelift-object`. The goal is to enable parallelization of compilation
when using `cranelift-object`. Currently this is difficult because
`ObjectModule::define_function` requires `&mut self`. This instead
soups up the `define_function_bytes` interface to handle relocations so
compilation can happen externally before defining it in a `Module`. This
also means that `define_function` is now a convenience wrapper around
`define_function_bytes`.
* 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`
* the target-lexicon crate no longer has or needs the std feature
in cargo, so we can delete all default-features=false, any mentions
of its std feature, and the nostd configs in many lib.rs files
* the representation of arm architectures has changed, so some case
statements needed refactoring
