Add more examples on the Rust API

[jubianchi]

Description

• Clean and comment the memory example (Add ability to cache Modules in memory #174)
• Move examples from wasmerio/docs.wasmer.io here:
  • Early exit (comments to add)
  • Error handling
  • Passing data (env)

Review

• Add a short description of the the change to the CHANGELOG.md file

[syrusakbary]

Suggested change

	//! A Wasm module can sometimes not work, it will produce errors in some
	//! cases.
	//! A Wasm module can sometimes be invalid or trigger traps, and in those case we will get
	//! an error back from the API.

[syrusakbary]

bors r+

[bors]

Build succeeded:

• Audit
• Code lint
• Test on linux
• Test on linux-aarch64
• Test on macos
• Test on windows