This commit speeds up the parsing/processing of ANSI escape codes by
roughly 7.5x. The speedup is mostly accomplished by replacing the regex
with dedicated parsing logic (nextAnsiEscapeSequence()) and reducing the
number of allocations in extractColor().
#### Benchmarks
```
name old time/op new time/op delta
ExtractColor-16 4.89µs ± 5% 0.64µs ± 2% -86.87% (p=0.000 n=9+9)
name old speed new speed delta
ExtractColor-16 25.6MB/s ± 5% 194.6MB/s ± 2% +661.43% (p=0.000 n=9+9)
name old alloc/op new alloc/op delta
ExtractColor-16 1.37kB ± 0% 0.31kB ± 0% -77.31% (p=0.000 n=10+10)
name old allocs/op new allocs/op delta
ExtractColor-16 48.0 ± 0% 4.0 ± 0% -91.67% (p=0.000 n=10+10)
```
Add String() methods to types, so they can be printed with %s. Change
some %s format specifiers to %v, when the default string representation
is good enough. In Go 1.10, `go test` triggers a parallel `go vet`. So
this also makes fzf pass `go test`.
Close#1236Close#1219
- Make structs smaller
- Introduce Result struct and use it to represent matched items instead of
reusing Item struct for that purpose
- Avoid unnecessary memory allocation
- Avoid growing slice from the initial capacity
- Code cleanup
