Translator which replaces known imports with snippets

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From: https://github.com/ewasm/ewasm-cleanup#libc-methods

Goal is to have a translator which takes a wasm module (called the "helper") and a list of imports. The current module is searched for the list of imports. If found, the import is removed and the function from the helper is copied in.

The first two to start with:

• memcpy(i32, i32, i32) -> i32 from https://github.com/ewasm/ewasm-cleanup/blob/master/memcpy.wast
• memset(i32, i32, i32) from https://github.com/ewasm/ewasm-cleanup/blob/master/memset.wast

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Fastly has a similar tool: https://github.com/fastly/lucet/tree/master/lucet-builtins/wasmonkey

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This tool is now independent: https://github.com/jedisct1/wasmonkey