wasm feature

[luizirber]

In sourmash I'm disabling default features to be able to use niffler in wasm environments. lzma and bzip2 require -sys crates, which are harder to compile properly in wasm32-unknown-unknown (browser) and wasm32-wasi (which does have a libc, but the toolchain still has issues with cross-compiling C code).

Pure rust implementations (like the one flate2 + miniz-oxide provide for gzip) are preferable in this case, because they will have less issues.

Having a wasm feature would be a shorthand for adding the formats that are supported.

Possible crates for adding more formats to wasm:

• https://github.com/gendx/lzma-rs (pure Rust crate for lzma)
• another lzma crate: https://github.com/PSeitz/lz4_flex
• bzip2 might be pure Rust someday, but still not there.