strict parsing by default

[shogo82148]

If it is after merging #116, there is no performance difference between Parse and ParseStrict.

goos: darwin
goarch: arm64
pkg: github.com/oklog/ulid/v2
               │   .old.txt   │              .new.txt               │
               │    sec/op    │   sec/op     vs base                │
Parse-10          9.158n ± 1%   6.051n ± 1%  -33.92% (p=0.000 n=10)
ParseStrict-10   14.515n ± 1%   6.106n ± 1%  -57.94% (p=0.000 n=10)
MustParse-10     10.095n ± 0%   7.076n ± 0%  -29.91% (p=0.000 n=10)
geomean           11.03n        6.394n       -42.03%

The reason for having Parse and ParseStrict separately disappears. Perhaps it would be acceptable to perform strict checks in Parse as well?