Major reordering.  Added scan_dimen/glue/mu, support for paragraphs...

Yes, this should have been many commits, but I was trying to get
\unravel{\lipsum\relax} to run seamlessly, and got carried away. The
main next step is boxes: presumably, this is enough to get
\unravel{\documentclass{article}\relax} to run.  This is necessary for
\setbox\strutbox...
Implement (with holes) commands 16 to 24.
Implement commands 25 to 31.  Mostly about boxes.
Bug fix of \@@_thing_use:nN.  Many unrelated minor tweaks.

When scanning something internal, the value should be converted to
the lowest of the obtained and target levels, not to the target
level as was previously done.
"Implement" all primitives.  Also a fix to scan_something_internal.

Many of the primitives are implemented with a "not-implemented"
error message only.
Since all primitives are "implemented", made some errors internal.
Replace all operations on the input gtl by some wrappers.

This will simplify changing the implementation of the operations
on this input string.
Tweak how the input primitive is implemented.

Apparently, the new version works better on some input (but I don't
understand why...).
The implementation of \endinput was wrong and broke nested inputs.
Optimize by changing the way the input is stored.