Precache, avoid cache-checking boolean values, and avoid unnecessary spreads

[tmcw]

Combination of two things that I could find that caused significant performance bumps for my benchmark:

• This optimizes the cache for literals. true, false, null, and undefined are always going to be equal to each other: we might as well fill the cache with them and when they're returned, avoid the cacheAndReturn logic.
• In hot code, using ... is causing performance drag because, in the current setup, zod's distributed files are using TypeScript helpers. This I'll file another PR for: my suspicion is that a very large portion of zod's performance problems are caused by that.

[tmcw]

I think removing the typeCache is a better solution than this one for primitives, and switching to es2018 is a better solution than avoiding spread syntax (for now).