fix: Add coercion for unsigned 64-bit types for bindings

[MaxGraey]

Example:

export function test(): u64 {
  return 18446744073709551614; // i64.const -2
}

Before:

assert(exports.test() == 18446744073709551614n) // -> fail due to return -2n

Now with this PR:

assert(exports.test() == 18446744073709551614n) // pass!

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