BigInt Support

[j-f1]

As part of #26, I created an API that uses BigInt values in some cases. They are not currently implemented, however. An important design question is whether bey should be represented as Int64/UInt64 — sacrificing precision for code size and ease of use — or whether some sort of BigInt library should be made a dependency so we can guarantee precision at the cost of code size and performance.

[kateinoigakukun]

js-api spec says BigInt is coerced to wasm types (i32, i64, f32, f64). So we don't need to care over 64bits case.