Smallvec

[dekellum]

Increases size of Element from 48 to 72 bytes, but also appears to increase perf:

test b00_round_trip_rcdom                ... bench:  17,145,188 ns/iter (+/- 2,406,106)
test b01_round_trip_marked               ... bench:  14,941,748 ns/iter (+/- 1,728,036)
test b11_decode_eucjp_parse_marked       ... bench:   2,858,239 ns/iter (+/- 472,707)
test b12_decode_windows1251_parse_marked ... bench:   2,269,685 ns/iter (+/- 295,803)
test b13_utf8_parse_marked               ... bench:   9,264,560 ns/iter (+/- 1,681,729)
test b20_text_content                    ... bench:      60,122 ns/iter (+/- 21,116)
test b30_text_normalize_content          ... bench:   1,224,610 ns/iter (+/- 354,732)
test b31_text_normalize_content_identity ... bench:     283,529 ns/iter (+/- 82,719)

test b40_marked_parse_only                ... bench:   9,375,353 ns/iter (+/- 1,015,046)
test b41_marked_clean                     ... bench:  10,779,967 ns/iter (+/- 2,863,567)
test b42_ammonia_clean                    ... bench:  12,525,043 ns/iter (+/- 2,755,911)