Fix foreach in TikZ to remember more stuff between iterations

[loopspace]

Motivation for this change

Fixes #356, #1047, #1303, #1313 by remembering more information between iterations of a foreach loop on a path, in particular the last node. Also provides a more flexible interface to remember more macros & dimensions at both the code level and the user level (the latter via pgfkeys).

See https://github.com/pgf-tikz/pgf/pull/1304 for a previous version of this pull request where something went wrong with signing the commits.

Checklist

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• [X] Code changes are licensed under GPLv2 + LPPLv1.3c
• [X] Documentation changes are licensed under FDLv1.2