Add date field check

[olorin]

Add basic "does this look like a date/datetime" check.

Working towards #113. No behavioural changes in warden yet, just adding groundwork.

@thumphries @erikd-ambiata

∃ ./dist/build/bench/bench field-parsing
benchmarking field-parsing/parseField/200
time                 24.51 μs   (23.52 μs .. 25.59 μs)
                     0.987 R²   (0.974 R² .. 0.997 R²)
mean                 24.22 μs   (23.72 μs .. 25.01 μs)
std dev              2.359 μs   (1.484 μs .. 3.796 μs)
variance introduced by outliers: 84% (severely inflated)

benchmarking field-parsing/checkFieldBool/boolean/100
time                 8.392 μs   (8.310 μs .. 8.512 μs)
                     0.997 R²   (0.993 R² .. 1.000 R²)
mean                 8.567 μs   (8.377 μs .. 9.339 μs)
std dev              1.112 μs   (394.4 ns .. 2.203 μs)
variance introduced by outliers: 92% (severely inflated)

benchmarking field-parsing/checkFieldBool/non-boolean/100
time                 7.005 μs   (6.849 μs .. 7.159 μs)
                     0.994 R²   (0.986 R² .. 0.998 R²)
mean                 7.108 μs   (6.928 μs .. 7.554 μs)
std dev              1.014 μs   (470.5 ns .. 1.908 μs)
variance introduced by outliers: 93% (severely inflated)

benchmarking field-parsing/checkFieldDate/date/100
time                 6.978 μs   (6.817 μs .. 7.140 μs)
                     0.993 R²   (0.983 R² .. 0.999 R²)
mean                 7.151 μs   (6.909 μs .. 7.941 μs)
std dev              1.372 μs   (253.3 ns .. 2.830 μs)
variance introduced by outliers: 96% (severely inflated)

benchmarking field-parsing/checkFieldDate/non-date/100
time                 10.55 μs   (9.938 μs .. 10.91 μs)
                     0.978 R²   (0.968 R² .. 0.986 R²)
mean                 8.945 μs   (8.446 μs .. 9.506 μs)
std dev              1.679 μs   (1.476 μs .. 1.863 μs)
variance introduced by outliers: 96% (severely inflated)

[thumphries]

🎋

[erikd-ambiata]

Probably want to make that const char * buf unless you intend to modify it.

[olorin]

Good point, thanks.

[erikd-ambiata]

Apart from a couple of places where parameters should be passed as const char* instead of char* and the idea of pulling the pre-condition test inside the function that requires it, 💯 .