Host functions use `Context::External` with a `*mut vm::FuncCtx`
pointer, casted to `*mut vm::Ctx`. It creates a conflict with exports
that also use `Context::External`.
This patch introduces `Context::ExternalWithEnv` to create a specific
path in the code for an external context with `*mut vm::FuncEnv`.
This patch fixes all the `linking.wast` tests in the spectests.
* Allow a module to have a different signature registry than the process-specific
* Add core ability to build compiled code caches
* Remove timing printouts
* Serialize/Deserialize memories to reduce copies
* Work more on api
* Relocate local functions relatively before external functions
* Fix incorrect definition in test
* merge errors caused by merge
* Fix emscripten compile
* Fix review comments
* Move wasmer-runtime to wasmer-runtime-core
* Add the runtime library
* Fix issue with macros using wasmer_runtime, fmt
* Make default compiler dependency optional
* Add instantiate and validate functions
