fix(parser)!: Make FnMuts take &mut I

[epage]

I expected this to have worse performance...

This is a step towards #72

v0.4.7

json/basic/canada       time:   [10.272 ms 10.308 ms 10.347 ms]
json/verbose/canada     time:   [32.539 ms 32.714 ms 32.893 ms]
json/dispatch/canada    time:   [8.9747 ms 8.9926 ms 9.0118 ms]
json/streaming/canada   time:   [10.612 ms 10.667 ms 10.726 ms]

#277

json/basic/canada       time:   [9.6649 ms 9.6840 ms 9.7037 ms]
json/unit/canada        time:   [9.0364 ms 9.0944 ms 9.1513 ms]
json/verbose/canada     time:   [31.032 ms 31.072 ms 31.114 ms]
json/dispatch/canada    time:   [8.5341 ms 8.5538 ms 8.5749 ms]
json/streaming/canada   time:   [10.679 ms 10.707 ms 10.739 ms]

#278

json/basic/canada       time:   [10.700 ms 10.779 ms 10.865 ms]
json/unit/canada        time:   [8.8489 ms 8.8636 ms 8.8791 ms]
json/verbose/canada     time:   [33.240 ms 33.291 ms 33.344 ms]
json/dispatch/canada    time:   [8.8891 ms 8.9045 ms 8.9203 ms]
json/streaming/canada   time:   [9.9510 ms 9.9905 ms 10.033 ms]

[coveralls]

Pull Request Test Coverage Report for Build 5476660644

• 333 of 463 (71.92%) changed or added relevant lines in 11 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-1.3%) to 49.196%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/combinator/parser.rs	6	7	85.71%
src/trace/mod.rs	0	1	0.0%
src/token/mod.rs	32	37	86.49%
src/binary/bits/mod.rs	29	38	76.32%
src/combinator/core.rs	32	44	72.73%
src/binary/mod.rs	69	91	75.82%
src/combinator/multi.rs	61	98	62.24%
src/ascii/mod.rs	87	130	66.92%

Files with Coverage Reduction	New Missed Lines	%
src/token/mod.rs	1	86.0%
src/combinator/parser.rs	3	59.34%

Totals
Change from base Build 5476653262:	-1.3%
Covered Lines:	1193
Relevant Lines:	2425

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💛 - Coveralls