Use primitive numbers as keys instead of `String` objects in Wasm execution context HashMap

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Is your feature request related to a problem? Please describe.

As part of PR #41, @ereslibre suggested this change: #41 (comment)

Describe the solution you'd like

Using primitive numbers instead of String objects will avoid all clone() operations needed right now. So it will use less memory and copy primitives will consume less CPU.

So far, we were using 8 hex chars, so 16^8 = 4.294.967.296 combinations. This is precisely the same that using a u32.

Describe alternatives you've considered

No response

Additional context

No response