Add wasm32-unknown-unknown compile target

[BelfordZ]

Currently we cannot compile to web assembly due to downstream dependencies still using deprecated crates.

• [x] Bump bind-gen to version that no longer uses rustc-serialize for rocksdb fork

• [x] #285

• [x] rust-num/num#366 -> Duplicate of -> rust-num/num#357

• [x] r8d8/rust-scrypt#4

• [ ] DaGenix/rust-crypto#437 -- fork available - Using less than optimal fork in the current PR. Take a look at this issue, seems like https://github.com/RustCrypto is the better way to go here.

• [ ] rust-bitcoin/rust-bitcoin#96 -- can we remove this? alternate mnemonic implementation?

Once all the above are resolved, we should be good to go for wasm targeting.

[newpavlov]

Note that instead of rust-scrypt you can use pure-Rust scrypt crate.

[mersinvald]

For rust-bitcoin story is a bit more complicated then just bip39 mnemonics: we use bip32 implementation from there and some constants.

[pajasevi]

Is there any way to compile to WASM with some workarounds? Is this gonna be resolved in near future?