perf(render): Use a Write

[epage]

render_to uses an io::Write. This is hopefully better than string
concatenation, especially if you can write directly to file. render is
still available for convinience (though it no longer returns an
Option).

Fixes #187

BREAKING CHANGES: Changed focus to io::Write

• Renderables must implement render_to.
• render no longer returns an Option.

[epage]

Before:

test bench_parse_template ... bench: 25,826 ns/iter (+/- 1,689)
test bench_parse_text ... bench: 634 ns/iter (+/- 52)
test bench_parse_variable ... bench: 2,527 ns/iter (+/- 182)
test bench_render_template ... bench: 7,225 ns/iter (+/- 2,762)
test bench_render_text ... bench: 262 ns/iter (+/- 20)
test bench_render_variable ... bench: 1,392 ns/iter (+/- 112)

[epage]

After

test bench_parse_template ... bench: 25,657 ns/iter (+/- 2,238)
test bench_parse_text ... bench: 629 ns/iter (+/- 37)
test bench_parse_variable ... bench: 2,562 ns/iter (+/- 203)
test bench_render_template ... bench: 7,600 ns/iter (+/- 726)
test bench_render_text ... bench: 248 ns/iter (+/- 105)
test bench_render_variable ... bench: 1,368 ns/iter (+/- 101)

Note: no real allocation change was made to the benchmark. It still calls render rather than render_to. In addition, it doesn't have many allocation heavy code paths.

[emoon]

Nice. Did you consider taking a generic parameter for Write? Now it's a trait object so that adds some extra overhead but perhaps it doesn't affect that much.

[epage]

It wouldn't have worked with how we track Renderables. They are boxed, so we need an object-safe function which means dyn Write rather than impl Write.

[emoon]

Ah, I see