'minutes function converts hh:mm symbol back to minutes

A symbol matching \d+:\d+ is assumed as being hours and minutes and
pushed as the corresponding number of minutes on the stack. The
'hours symbol has a function to convert a number of minutes back
to hours:minutes.

Also, to support the above, a function-symbols functions in lingo.

This seems more logical, in particular as the nil argument is a
valid character sequence as well. There is no conflict with the
CLHS, as it does not specify an "all whitespace" argument.

Affected builtin functions are string-trim, string-left-trim, and
string-right-trim.

In the course, factored out and conflated the flesh of the code
from the three functions, which was very repetitive before.

 - shorten some function descriptions
 - cut off long lines with `[...]' so they fit into terminal width

Previously, substring end indexes beyond the end of the string were
considered an error, contrary to documentation. The related tests
confirmed that. Fixed at both places.

Incorporate the docstrings of functions imported directly from
lingo. Also, have an `apropos` command to search all function
descriptions.

Before, a missing etags caused a build error, and generating the
Lisp preload file and the function registration was first tried
with lingo, and on error done with the shell script. While the
corresponding "command not found" errors were ignored, they were
ugly.

Now, the build checks if etags is there, and if not, ignores that
more gracefully; more or less the same with a pre-existing (or not)
lingo. Much better.

Otherwise, terminal initialisation escape sequences are sent to
stdout even if it is not a terminal, and that is unfortunate, to
say the least. Or, looked at the other way, it is fortunate that
this botched the self-generation if Go source files, so the lingo
build failed -- in time for me to notice. Yay!

Also closes issue #1.

Saving a history file still doesn't work, but I guess I can live
with that. And it is not every day's evening yet.

The mode appliers (which are, I suspect, bogus anyway) were put in
dep/, where they were never used, as everything related to that
happens in lob/ports anyway. Eliminated.

Also, rename min-, min+ => time-, time+

min- to add times in hh:mm format,
min+ to add them,
2time to convert the resulting minutes back to hh:mm