Describe uvectors

[jpellegrini]

But we still don't give their length in the description.

@egallesio -- the %uvector-length procedure takes an integer (the tip) and a uvector. For describe, it would make sense to have a wrapper that does not need the tip, so we can also report the length.

What do you think is better?

1. Add a new procedure, like %general-uvector-length that only takes a uvector? Something that would only do

   return MAKE_INT(UVECTOR_SIZE(v));

   and just return the length?

2. Change %uvector-length to subr12, so it can be called with args (tip uvec) or (uvec)

3. Write a general API for uvectors that do not require the user to include the uvector type? That would include uvector-length, uvector-{ref,set}, uvector->list, uvector?... (The last one really makes sense - -SRFI 4 does not offer a general uvector? predicate... See that the code in this PR uses is-a?)

4. not report the length

5. Some other possibility?

[egallesio]

Hello @jpellegrini, Sorry for the long delay. But finally here it is. The commit is a mix of point 2 and 3.