Remove the Context type from wasmtime (#825)

* Remove the `Context` type from `wasmtime`

This hasn't really ended up being used in all that many places and the
dependencies on it were pretty minimal. This commit removes it in favor
of simplifying its various users a bit and/or leveraging the
`Engine`/`Store` structures where possible.

* Run rustfmt

crates/api/src/lib.rs

@@ -7,7 +7,6 @@
 //! itself for consumption from other languages.
 
 mod callable;
-mod context;
 mod externals;
 mod instance;
 mod module;