Add compiler features to wai-bindgen-wasmer

[kajacx]

Description

Specify which compiler you want to use with a feature directly in wai-bindgen-wasmer .

Current situation:

When you want to use wai-bindgen-wasmer with the sys feature, you have to separately include wasmer to specify which compiler you want to use, like this:

wai-bindgen-wasmer = { version = "=0.4.0", features = ["sys"]}
wasmer = {version = "=3.3.0", features = ["cranelift"]}

Why this is suboptimal:

There are several reasons:

• You have to remember to do it. New people who just want to use wai-bindgen-wasmer would not know this and would be confused by the compiler error.
• You have to track down which specific version of wasmer to use.
• You have the decision fatigue of whether to use wai-bindgen-wasmer::wamser or just wasmer in your code.
• Your cargo setup is needlessly complicated when you are building for both native and web targets.

With the proposed change:

You just specify which compiler you want to use directly in the wai-bindgen-wasmer dependency:

wai-bindgen-wasmer = { version = "=0.4.0", features = ["sys", "cranelift"]}

Things to consider:

Should cranelift be enabled by default when using sys? It is when using wasmer directly, but I don't think that would fit here well. Alternatively, sys could be removed in favor of sys-cranelift, sys-singlepass and sys-llvm instead.

Also, is there a changelog for the wai-vingen-wasmer crate, or should I put this change to the main changelog?