Figuring out which boolean settings go into each preset is not easy by
inspecting the DSL source (e.g. meta/src/isa/x86.rs). This patch extends
the comments in the Rust that's generated by that DSL to list the names
of the settings together with the name of the preset.
This commit adds the `wasmtime settings` command to print out available
Cranelift settings for a target (defaults to the host).
The compile command has been updated to remove the Cranelift ISA options in
favor of encouraging users to use `wasmtime settings` to discover what settings
are available. This will reduce the maintenance cost for syncing the compile
command with Cranelift ISA flags.
