Relax unshadowing for let defs

[bugarela]

Hello

This makes unshadowing smarter so it doesn't have to rename all names, only the ones that are actually shadowing another name.

Unfortunately I could just do it for the let definitions, not for lambdas. See #1443. But this is the most important part, because it fixes names for nondets and nondetPicks looks good (see the test).

I found a couple of problems in our definition collection through testing this, since we rely on more collected metadata to decide what to unshadow now. So there are some small fixes for those as well.

• Tests added for any new code
• Documentation added for any new functionality
• Entries added to the respective CHANGELOG.md for any new functionality
• Feature table on README.md updated for any listed functionality

[p-offtermatt]

LGTM, great change.
It's really important to have the variable names for the nondets fixed and predictable to consume these in another program, so this is great to have.