AVL Interval Mapping & Unicode Alphabet Replacement

[srhickma]

This pr makes several changes:

• Make ranges operate on Unicode character ranges, rather than alphabet order. This allows ranges to be specified without dependence on the alphabet.
• Make alphabet optional. If an alphabet is not specified, any characters are allowed.
• Enforce that lexed characters must be present in the alphabet, if one is specified.
• Add Interval type and IntervalMap data structure, which can be used to map key-ranges to values, rather than single keys to values. IntervalMap is implemented as a modified interval tree, which is internally implemented as an AVL tree.
• Implement ranges in the TransitionTrie using an internal IntervalMap. This ensures that extremely large ranges can be stored without impacting performance. For example, in the previous implementation a range of 1000 characters would result in 1000 hashmap entries in the transition trie root node. Now, the entire range can be represented as a single Interval struct, and membership can be queried in O(log(n)) time (worst-case), where n is the number of ranges for a particular state.

closes #155

[codecov[bot]]

Codecov Report

Merging #184 into master will decrease coverage by 0.08%. The diff coverage is 93.89%.

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
- Coverage   95.11%   95.02%   -0.09%     
==========================================
  Files          26       27       +1     
  Lines        3295     3478     +183     
==========================================
+ Hits         3134     3305     +171     
- Misses        161      173      +12

Impacted Files	Coverage Δ
src/lib.rs	88.7% <ø> (ø)	:arrow_up:
src/core/data/mod.rs	100% <ø> (ø)	:arrow_up:
src/core/parse/mod.rs	93.15% <ø> (ø)	:arrow_up:
src/core/spec/region.rs	95.12% <100%> (ø)	:arrow_up:
src/core/fmt/pattern.rs	97.59% <100%> (+0.03%)	:arrow_up:
src/core/lex/longest_match.rs	97.43% <100%> (+0.06%)	:arrow_up:
src/core/spec/lang.rs	100% <100%> (ø)	:arrow_up:
src/core/lex/mod.rs	88.03% <69.56%> (-1.97%)	:arrow_down:
src/core/lex/ecdfa.rs	96.56% <92%> (-1.01%)	:arrow_down:
src/core/data/interval.rs	96.32% <96.32%> (ø)
... and 3 more

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