Make typings more specific

[GulgDev]

Hello! Currently, binaryen.d.ts contains definitions that don't allow TypeScript compiler to properly narrow down ExpressionInfo. It would be nice to have more specific typings. I suggest the following changes:

1. Change type of Id exports to match the corresponding enum value.
2. Change the ExpressionInfo interface to a union type with the corresponding ids.

Should I make a PR or is this behaviour intentional?