Handle change of `excl::.functions-defined.` from list to (no-values) hash-table.

Based on further discussions with Duane Rettig and Kevin Layer at Franz.

Previous attempt to fix #141 had accidentally broken ASDF on Allegro versions prior to 11.

This macro is incorrectly implemented and, to date, unused.  So
I am removing it from UIOP.

Extend the setter for UIOP:GETENV to do this.

Set the test to expect the check for setting an environment variable to NIL to fail on clasp.

Raise `not-implemented-error` if clasp tries to set environment variable to NIL.

This is necessary to avoid ASDF putting single- and multi-threaded SBCL fasls in the same directory.  That's the wrong thing, since they are not compatible. By giving such builds different implementation-identifiers, they are kept apart.

Add test that verifies the existence of issue 142.
Then fix the issue -- style warning when `with-temporary-file` does not use the pathname variable in code following the `:close-stream` form.

Necessary to build cleanly on Allegro v 11 (available now in beta).

A number of tests that were disabled for older ABCL versions are enabled now for more recent releases.

Expand on the use of :UNINTERN keyword argument, based on discussion
around issue #85.

Eric Timmons authored Jan 10, 2022 and Robert Goldman committed Jan 10, 2022

Signal a style warning if we `:import-from` using a package that does not exist and an empty symbol list. Remains an error to `:import-from` a package that does not exist with a non-empty symbol list.

Add some tests to try and ensure `:uiop/package` and `:uiop/package-local-nicknames` are not modified.

Also add brief docstring to tell programmers limits on its use.

Resolve "UIOP:DEFINE-PACKAGE signals a TYPE-ERROR when trying to IMPORT-FROM a non-existent package" (#98)

The code handling other options to `define-package` used `find-package*`, 
which (by default) errors if it can't find the package it's looking for.  The
exception was the code for `:import-from`. This caused cryptic error messages
(issue #98) when trying to import from a package that did not exist.

Switched to using `find-package*`, added a new exception class, and added a test.

This is implemented in the most algebraically abstract way (as the conjunction of two calls to `version<=`).