Rewrite code for parsing Tcl config files

[htfab]

The current implementation for parsing Tcl config files fails to catch environment variables defined in subordinate .tcl files included from the main .tcl with the source command. This breaks openlane1 compatibility for almost every user design submitted to Tiny Tapeout.

Tcl config files are parsed using TclUtils._eval_env(). The current version

• modifies os.environ to provide inputs to the script
• runs the script
• converts ::env to numbered variables on the tcl side
• rolls back the changes to os.environ
• reads the numbered variables from tcl back into python
• filters the results to entries appearing in set ::env(...) constructs in the script

The last step is there to filter out entries inherited from the environment, but it also filters out entries defined in included files. This python-tcl interface relying on os.environ modifications and rewriting ::env to numbered variables also feels brittle in some other ways. I'm proposing to rewrite it in the following manner:

• set env(...) directly instead of modifying and rolling back os.environ
• provide a python callback function to tcl code for easier backwards communication
• patch tcl's set function to also call our callback
• on the python side, save all set calls that match env(...) to the returned dictionary

This makes the code more robust and only saves environment variables explicitly set by the Tcl config, making it unnecessary to use the problematic filtering step.

[donn]

Merging to main. Rationale: fixes incompatibility with OpenLane 1 config files.

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Misc Enhancements/Bugfixes

• Overhauled Tcl configuration loading code to fix a number of bugs that may occur when a Tcl file sources another Tcl file, such as for TinyTapeout-based configs (thanks @htfab)