Hi y'all,

This patch adds support for a recent draft SRFI number 253
https://srfi.schemers.org/srfi-253/ (authored by me)

I'm pretty sure I messed up a lot of things convention-wise, so feel
free to edit and point out if something looks off. In particular:
- There's no copyright notice in the new files, because I wasn't sure
  whether I should include any, and, if so, who to attribute copyright
  to. I'm fine with any GPL license if you ask me.
- I've added more than one @cindex terms to the manual section. Is that
  okay? They all are mostly relevant.

An expertise of someone familiar with types and optimization in Guile
won't hurt either, because I'm using GOOPS for type checking, which
likely causes a huge performance overhead. I'm sure there are faster and
stricter ways to check/enforce types. SCM_IS_A_P something? Is my assume
macro (what about adding support for SRFI-145, by the way?) going to
help type inference engine/optimizer or is it too noisy?