v0.2.0

The C interface. Nothing a Zig caller sees changed, and every input parses to
what it did in 0.1.0.

Added

• A C interface, for every language that can call C. zig build lib builds
  libdateparsa, static or with -Dlinkage=dynamic shared, and installs
  include/dateparsa.h. It covers parse, detect, layout reuse, Parser,
  strict mode and its diagnostics, locales by bit or by BCP 47 tag, RFC 3339
  output and Unix seconds, and allocates nothing: every struct is the
  caller's. What a C caller passes in is checked, not trusted: a base time
  that is no date, an offset of a day or more, a base year more than
  999,999,999 from year 0, a locale bit the build left out, a NULL, or a
  layout or parser the library never filled in is
  DATEPARSA_INVALID_ARGUMENT.