fix Issue 13252 - ParameterDefaultValueTuple affects other instantiations

Expression::equals should also compare operand types.
fix Issue 13225 - [ICE] Access violation on invalid mixin template instantiation
Remove redundant error
fix Issue 13245 - segfault when instantiating template with non-compiling function literal
Remove toHeadMutable call from NullExp::equals and ArrayLiteralExp::equals

I'm not sure we should also remove toHeadMutable call from IntergerExp, RealExp, ComplexExp, and VarExp::equals. To avoid accidental existing code breaking, I'd leave them in 2.066 beta/rc phase.
Merge pull request #3846 from 9rnsr/fix13252

[REG2.061] Issue 13252 - ParameterDefaultValueTuple affects other instantiations
Fix comparisons functions, likely fixes issue #8596.
Merge pull request #3840 from 9rnsr/fix13225

Issue 13225 - [ICE] Access violation on invalid mixin template instantiation
fix Issue 13237 Wrong code with -inline -O
fix Issue 13235 - Wrong code on mutually recursive tuple type
Merge pull request #3848 from WalterBright/fix13237

fix Issue 13237 Wrong code with -inline -O
[Refactoring] Remove unused methods appendExp and appendState
[Refactoring] Remove unused fields
Merge pull request #3849 from 9rnsr/fix13235

Issue 13235 - Wrong code on mutually recursive tuple type
Merge pull request #3850 from 9rnsr/refactor_unused

[Refactoring] Remove unused code
Merge pull request #3844 from Safety0ff/wconv

Fix comparisons functions