Remove date regexp

[pelletier]

Issue: #440

Hand-roll the date matching logic to avoid trying to match a regexp on
every integer.

benchmark                    old ns/op     new ns/op     delta
BenchmarkUnmarshalToml-8     293449        272134        -7.26%

benchmark                    old allocs     new allocs     delta
BenchmarkUnmarshalToml-8     2746           2650           -3.50%

benchmark                    old bytes     new bytes     delta
BenchmarkUnmarshalToml-8     133604        127548        -4.53%

A lot of the new code looks duplicated. It's not quite. I found it easier to read and more precise in the reported errors than the various refactors I tried.

@moorereason

[codecov]

Codecov Report

Merging #447 into master will increase coverage by 0.23%.
The diff coverage is 98.46%.

@@            Coverage Diff             @@
##           master     #447      +/-   ##
==========================================
+ Coverage   93.32%   93.56%   +0.23%     
==========================================
  Files          10       10              
  Lines        2158     2253      +95     
==========================================
+ Hits         2014     2108      +94     
- Misses        104      105       +1     
  Partials       40       40              

Impacted Files	Coverage Δ
token.go	100.00% <ø> (ø)
parser.go	90.33% <91.30%> (-0.34%)	⬇️
lexer.go	98.91% <100.00%> (+0.22%)	⬆️

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[moorereason]

LGTM 👍