Use variable width integer encoding for compression.
Use variable width integer encoding for compression.

This slightly modifies the implementation in #226 to use variable width
integers. This saves a touch more space with regexes with huge
alternations spanning larger-than-127 instructions (typically from Unicode
character classes).

This also adjusts `approximate_size`, which is the actual gatekeeper
behind deciding whether the DFA has filled its cache or not. The approximation
is now reduced slightly to account for the space savings.

The variable integer encoding used is from Protocol Buffers, documented
here: https://developers.google.com/protocol-buffers/docs/encoding#varints

golddranks/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang/regex,SeanRBurton/regex,rust-lang-nursery/regex,SeanRBurton/regex,rust-lang/regex,rust-lang-nursery/regex,SeanRBurton/regex,SeanRBurton/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,golddranks/regex,golddranks/regex,rust-lang/regex,SeanRBurton/regex