Parsing rewrite

[dnfield]

This is close to ready.

Fixes #47 - this code should not retain any unnecessary references during parsing, and should allow for lower memory usage.

[coveralls]

Pull Request Test Coverage Report for Build 280

• 473 of 502 (94.22%) changed or added relevant lines in 7 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.8%) to 80.351%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/src/svg/parsers.dart	24	27	88.89%
lib/src/vector_drawable.dart	22	25	88.0%
lib/src/svg/parser_state.dart	355	378	93.92%

Files with Coverage Reduction	New Missed Lines	%
lib/src/vector_drawable.dart	2	81.95%

Totals
Change from base Build 255:	0.8%
Covered Lines:	1235
Relevant Lines:	1537

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💛 - Coveralls

[dnfield]

Green!

Still a couple minor things to shift around, and I'd like to wait to merge this until the xml package is available on pub instead of just a git hash.

Fixes #47