With the ~--eval~ option you can evaluate an expression in mu/scm
environment. For example:

$ mu scm --eval \
  '(format #t "found ~d match(es)\n" (length (mfind "hello")))'
found 7173 match(es)

Add command-line parameter and implement.

Example was still using the obsolete name.

Fixes #2891

Some of the tests were using (mfind "") to find messages, but since some
messages don't have dates the order is implicitly defined by the directory
order. This usually works, but not always (i.e. guix).

So let's make the behavior deterministic.

Fixes #2888

Fixes #2889.

For a store, so to avoid assoc-ref.

Some methods that returned lists would return ~#f~ when there were no
results (e.g., to, from, cc, bcc, references, labels); however, it's
nicer to return an empty list instead, which what we now do. Technically a
breaking change, but justified while we are still 'experimental'.

Add a method personal? to check if some string looks like a personal address;
add docs / tests as well.

Rename the all-labels method into simply 'labels'

Make some define* into define-method, for consistency.