Improvements to Options

[vlovich]

Currently there's no way to construct the Options struct because Compression and Cache are in private module. AFAICT there's no way to request SNAPPY compression.

Additionally, it would be nice if there were separate read / write options to make it clearer which options apply to TableBuilder vs TableReader.

[dermesser]

w.r.t. your first point - doesn't https://docs.rs/sstable/latest/sstable/enum.CompressionType.html do what you need to enable compression? The cache itself is not supposed to be set by the user, but configured using the with_cache_capacity() constructor function.

About your second point, I agree. It should be reasonably easy to split the Options struct.