Improve performance

[sindresorhus]

Fixes #10

Apparently, creating a vm context is very expensive. It now reuses the context. This should in theory be safe as JavaScript is single-threaded, so two functions cannot execute, for example, firstMatch at the same time. But it does make me think whether there's anything I'm missing.

// @fityannugroho

---

❯ nr benchmark

> super-regex@0.2.0 benchmark
> node benchmark/index.js

str.matchAll: 0.236ms
matches: 2.355ms
matches-t500: 16.928ms
str.match: 0.149ms
firstMatch: 2.334ms
firstMatch-t500: 22.152ms

	Count chars: 23749
	Count rows: 405
	Count matches: 363 363 363 363 363 363

[fityannugroho]

I have tested it on my own device, and I think it is already fast enough.

str.matchAll: 0.663ms
matches: 10.594ms
matches-t500: 84.115ms
str.match: 0.469ms
firstMatch: 10.175ms
firstMatch-t500: 106.647ms

        Count chars: 23749
        Count rows: 405
        Count matches: 363 363 363 363 363 363