restore special case parsing of numbers for in memory JSON

[KristofferC]

I was trying to figure out why the from and to variables where threaded through the number parsing routines since it seemed like they were always 1 and length(ps.utf8array) respectively. Turns out there is a special cased number parser for in-memory JSON that is being failed to be dispatched to after the introduction of ParserContext (#224 cc @kmsquire).

This restores the special case for a nice performance boost:

using JSON
using BenchmarkTools

a = string("[", join([rand(Int) for i in 1:10^3], ","), "]");
@btime JSON.parse($a)

Before:

julia> @btime JSON.parse($a)
  246.210 μs (1016 allocations: 32.33 KiB)

After:

julia> @btime JSON.parse($a)
  89.930 μs (1011 allocations: 32.11 KiB)

Arguably the parse_number(pc::ParserContext, ps::ParserState) should be changed to parse_number(pc::ParserContext, ps::StreamingParserState) to avoid this happening again.

[TotalVerb]

LGTM. Not sure why the codecov is failing, since this improves coverage.

[KristofferC]

Coverage is fubar...