Implement new Context API for Wasmer 3.0

[Amanieu]

Overview

This PR reworks the way Wasmer manages objects used by instances, specifically: Instance, Memory, Table, Function, Global and ExternRef. Previously these were tracked using reference counting, but this cannot work fully because Wasm code can manipulate references to functions (and by implication the instance that function is from) using tables.

The new approach uses a Context type which owns all of the objects listed above: objects are only freed when the Context is dropped. The Instance, Memory, Table, Function, Global and ExternRef types are now simply wrappers around an integer which indexes a vector in the Context. As a result, any function on those "handle" types now need to take a &Context or &mut Context as a parameter to access the actual data in the context. This also vastly simplifies Wasmer's thread-safety story: since all mutating operations require a &mut Context, no synchronization or internal locks are needed.

Additionally, the way host functions are represented is also changed. The WasmerEnv trait is removed since it was unergonomic and hard to use correctly. Instead, the state for host functions is stored directly as the T in Context<T>. This state is initialized when the context is created, and can be accessed through the data and data_mut methods on Context<T>. All host functions receive a ContextMut<'_, T> as their first argument, which gives them access to the host state of the context. This is much better than the old approach since it provides mutable access without locks and allows all host functions to share the same state.

This design is heavily inspired by this RFC from Wasmtime.

TODO

• Update Wasmer examples to the new Context API #2985
• Update all tests to use the new Context API #2908
• Port C API to new Context API #2973 Upgrade c-api to use the new API
• Update wasmer-wasi to use the new API
• Update wasmer-emscripten to use the new Context API #2909
• Update wasmer-cli to use the new Context API #2913
• Migrate JS to the new Context API #2912
• Update wit-bindgen to use the new Context API #2911 (Non blocking)
• Update Wasmer 3.0 migration guide with the new Context API #2910

Fixes #2893
Fixes #2873
Fixes #2866
Fixes #1840
Fixes #1734
Fixes #1632
Fixes #1522
Fixes #1630
Fixes #2838
Fixes #2856
Fixes #2544
Fixes #2816

[epilys]

bors r+

[bors]

Build succeeded:

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