Fast markdown renderer

[rumpl]

Benchmark	Fast Renderer	Glamour	Improvement
Full Document
Time	206μs	2,415μs	11.7x faster
Memory	90KB	1,002KB	91% less memory
Allocations	1,231	83,845	99% fewer allocations
Small Text
Time	1.2μs	96μs	78x faster
Memory	1KB	43KB	98% less memory
Allocations	22	3,412	99% fewer allocations
Code Block
Time	60μs	227μs	3.8x faster
Memory	24KB	78KB	69% less memory
Allocations	342	3,006	89% fewer allocations

[rumpl]

I gated this behind a CAGENT_EXPERIMENTAL_MARKDOWN_RENDERER environment variable, let's merge and fix things as we see them

[rumpl]

Left -> ours
Right -> glamour