perf: use caches to speed up output generation

[andyburris]

Really only noticeable for huge terminals so hard to use the current benchmarks, but anecdotally on my 150x200 terminal it's sped renders up from ~40ms to ~9ms, so almost 5x faster. Presumably it'd be more of a speedup the larger the terminal gets.

All tests (that were passing on current master) are passing.

[sindresorhus]

Output is recreated on every render, so it's unclear to me why this actually improves performance, unless the output has duplicated lines.

[andyburris]

I think because there's a lot of redundant processing even within a single output frame. For example:

• widestLine is internally calling stringWidth on every line, just to do that again manually later on
• more importantly, stringWidth is getting re-run on every individual character