decimal128.rs should have cfg wrappers for its methods. for wasm target, everything but `from_raw_bytes` can just be `unimplemented` for now.

i forsee a bunch of issues with bson crate and supporting two different types of Decimal128. There is a chance that its own wrapper would have to be implemented as a generic to get this to work, or alternatively in this branch, we should just get rid of the other decimal128 implementation so we can at least compile without errors.