Turn ewasm_api into a trait

[ewasm]

Turn ewasm_api into a trait

[jakelang]

@axic I think that EEI as a trait doesn't make sense. I would bottle it up into a module so that it can still be isolated in its own namespace, but since it isn't an interface for a type it shouldn't be a trait.

As a module, EEI methods can be called like ewasm_rust_api::eei::function(args) without being associated to a type.

[axic]

Whatever is the right Rust term, it would make sense to have the ability to swap out the underlying implementation with preloaded data to enable proper in-Rust testing of the precompiles/runevm.

[jakelang]

@axic I want #6 to be merged before this is implemented, to avoid nasty diffs and merge conflicts.

[axic]

Here's some initial work from August: https://github.com/ewasm/ewasm-rust-api/tree/turn-into-trait