create readonly variables
create variables marked as read-only, applicable to scalar, list, hashes
and all sorts of recursive data-structures.

Use as an alternative to constant, the main difference is that Readonly
creates scalars and other variables that are much easier to interpolate
into strings and other contexts.

p5-Readonly auto-detects the arch-dependent package p5-Readonly-XS,
which is faster, and uses it if available.

Maintainer: The OpenBSD ports mailing-list <ports@openbsd.org>
