This will be needed to later support zero-copy deserialization of independent
syntax sets, but is interesting and useful on its own.
Instead of deferring serialization and deserialization to syntect, we implement it
ourselves in the same way, but make compression optional.
We can't use #[from] on Error::Msg(String) because String does not implement Error.
(Which it shouldn't; see e.g. https://internals.rust-lang.org/t/impl-error-for-string/8881.)
So we implement From manually for Error::Msg, since our current code was written
in that way for error-chain.
Move code to build assets to its own file. That results in better modularity and flexibility.
It also allows us to simplify HighlightingAssets a lot, since it will now always
be initialized with a SerializedSyntaxSet.
