Performance: Cache missingSupport and refactor Detector

[cmrn]

This PR improves the performance of doiuse in two ways, making it run 3-10 times faster:

1. Cache the result of BrowserSelection.missingSupport(), since compiling the list of missing features is an expensive operation. This improves performance when the postcss plugin is run over multiple CSS files, like when linting an entire codebase.
2. Refactor Detector to flatten and normalise all feature checks during initialization, rather than using expensive Object.entries operations to parse the rules at every node in the syntax tree.

Benchmarks (from master...RJWadley:doiuse:benchmark-2):

run	time	diff
Before changes	5.3s	-
Cache missingSupport	4.8s	-0.5s
Refactor of Detector	2.2s	-3.1s
After both changes	1.7s	-3.6s

Other benchmarks:

benchmark	before	after
postcss/benchmark	233ms	44ms
stylelint's benchmark	6s	1.5s
private repo: ~300 CSS files	50s	5s
private repo: ~8300 CSS files	8m 50s	1m 30s

[clshortfuse]

Thanks for the PR. It all looks good. Just left some comments in case some improvements can be made.

[cmrn]

hi @clshortfuse, just a friendly bump on this PR :)