fix Issue 9374 - 'super' should be accessible inside template constraint
small refactoring
fix Issue 6538 - ICE(mangle.c) Invalid template constraints
fix Issue 9361 - Nasty bug and/or error message for template constaints using this

Add test cases
Merge pull request #1537 from WalterBright/b28

small refactoring
Merge pull request #1539 from 9rnsr/fix6538

Issue 6538 - ICE(mangle.c) Invalid template constraints
fix Issue 9369 - DDoc hardcodes '&' -> '&amp;' in code
Merge pull request #1541 from WalterBright/fix9369again

fix Issue 9369 - DDoc hardcodes '&' -> '&amp;' in code
hide private/package module level symbols

- restrict visibility of module level symbols to given access rights

- restrict access rights when searching across imported modules
adapt search_correct

- find private symbols
- take first symbol for ambiguous proposals
- don't report errors for Scope::search_correct
more verbose search correct proposals

- print protection and kind
- print fully qualified name
partial-fix of #143
fix issue 1238
fix issue 1754
fix issue 2991