This commit does two things: 1) it fixes `wasmtime_rust::wasmtime` proc macro by
adding the missing import to the `__rt` module, and fixing the scoping inside
the macro itself; and 2) it augments the `wasmtime_rust::wasmtime` proc macro with
custom error messages in case the implementor forgets the `self` argument in the
trait methods.
This commit is an attempt to reduce the number of crates necessary to
link to when using `wasmtime::Config` in "default mode" or with only one
or two tweaks. The change moves to a builder-style pattern for `Config`
to only require importing crates as necessary if you configure a
particular setting. This then also propagates that change to `Context`
as well by taking a `Config` instead of requiring that all arguments are
passed alone.
