Get rid of common crate

[alexpovel]

It's always tempting to have it, but it's also a smell. A first step was taken in 5664118 , using itertools' powerset.

Remaining items are:

• instrament: can be moved back into core, not used anyway else currently anyway
• strings.titlecase: looked around, as it seems very easy for there to be a crate for it, but no dice (funny that this is so "hard"):
  • https://crates.io/crates/capitalize: does ascii_lowercase, which we cannot use
  • https://crates.io/crates/roe: looks good and professional, but it's work in progress, and as of 0.0.4 only has lower- and uppercasing, no titlecasing
  • https://crates.io/crates/titlecase: specifically ignores words, which we cannot do
  • https://crates.io/crates/unicode_titlecase: looks promising, similar to my implementation anyway
• binary_search_uneven: currently only lives externally because of benchmarks, as Criterion benchmarks can only use the public API
• is_compound_word: small function but unlikely to find a suitable crate for that. Lives externally because build.rs prepares the word list using that same algorithm (so that the processed word list doesn't contain compound words, as that would be wasted space)

[alexpovel]

binary_search_uneven could maybe be useful enough to be its own crate, but is_compound_word isn't. However, it looks like we cannot have these external/local dependencies on common when publishing. These items would need to be their own crate:

• Publishing a crate with unpublished dependencies rust-lang/cargo#6738
• Add support for path dependencies in crates.io uploads rust-lang/cargo#1565

[alexpovel]

• instrament done in 7916ed7
• strings.titlecase done in 2c5c8e5
• binary_search_uneven done by building an external create for it (but not using it as fst is better suited)
• is_compound_word done in c640363; by building an external crate as well