Minimize the use of DUMMY_SP

[farcaller]

Error messages from PT are not that useful as most of them is based on DUMMY_SP.

• figure out how to provide spans for generated code
• clean up most (all?) of the DUMMP_SP uses

[mcoffin]

Mentioned in #298 already but for anyone that finds this...

Created lint plugin for checking for the usage of DUMMY_SP in syntax extensions.
https://github.com/mcoffin/syntaxext_lint

[mcoffin]

#298 landed fixing this at least within the ioreg scope. Still need to refactor platformtree but not going to do it now pending massive updates or a rewrite.