Improve performance

[alecthomas]

I'm now largely happy with the design and implementation of Participle, so it's time to start thinking about performance.

$ go test -v -bench=. ./
goos: darwin
goarch: amd64
pkg: github.com/alecthomas/participle/_examples/µc
BenchmarkParser
BenchmarkParser-12    	    163	  7393469 ns/op	   674820 B/s	5350378 B/op	  83749 allocs/op
PASS
ok  	github.com/alecthomas/participle/_examples/µc	1.958s

The parser generates a lot of garbage now, and throughput is not great.

[alecthomas]

The GraphQL parser is 5x faster:

➜ ~/…/_examples/graphql go test -bench=. .
goos: darwin
goarch: amd64
pkg: github.com/alecthomas/participle/_examples/graphql
BenchmarkParser-12    	   2572	   407658 ns/op	  2582288 B/s	 198843 B/op	   3198 allocs/op
PASS
ok  	github.com/alecthomas/participle/_examples/graphql	1.107s

2.5MB/s vs. 670KB/s. Could be the lookahead required in uc?