Support compiling to WASM

[jsdw]

I suspect this is already possible, but let's make sure, and let's add a CI step to ensure that wasm builds don't break going forwards.

[gilescope]

cargo build --target wasm32-unknown-unknown seems to suggest it's not quite:

   Compiling scale-value v0.1.0 (/home/gilescope/git/scale-value)
error[E0599]: the function or associated item `decode` exists for struct `BitVec<u64>`, but its trait bounds were not satisfied
   --> src/scale_impls/decode.rs:300:79
    |
300 |         (BitOrderTy::U64, BitStoreTy::Lsb0) => to_bit_sequence(BitVec::<u64, Lsb0>::decode(data)?),
    |                                                                                     ^^^^^^ function or associated item cannot be called on `BitVec<u64>` due to unsatisfied trait bounds