type with same name in different scope now works

[timotheecour]

• fix regression(1.2 => 1.4 onwards): Block-local ref types with the same name lead to bad codegen: no member named #15526
• fix regression(1.2=>1.4 onwards): enum with same name in different scope gives wrong RT results #17162
• fix Redefinition and usage of ref type in different scopes leads to invalid c code #17666
• fix tests/enum/tenum.nim which was wrong (was affected by regression(1.2=>1.4 onwards): enum with same name in different scope gives wrong RT results #17162)

unrelated question for reviewer

sighashes doesn't seem to do any caching, is that intentional?

future work

• avoid (pre-existing) hacks to prevent endless recursion, and use caching via a hash table, which should also speed hashType (but we also need to assess whether hashType has some non-negligible cost)