Improve Interval parsing performance

[omus]

Interval parsing support was added in #128 using regular expressions. Usually we can get better performance and reduced allocations if we write a lower-level parser instead of using general regular expressions. The current performance is:

julia> using Intervals, BenchmarkTools

julia> @btime parse(Interval{Int}, "[1,2)")
  734.102 ns (13 allocations: 656 bytes)
Interval{Int64,Closed,Open}(1, 2)