CTFE: Refactor sliceAssign from assignCommon

Split the absurdly large interpretAssignCommon into smaller pieces
CTFE: Refactor assignToIndex from assignCommon

Split the absurdly large interpretAssignCommon into smaller pieces
CTFE: Fix indentation
Fix bug 9634 [CTFE] wrong code concatenating arrays of structs

When concatenating, the array literals need to be duplicated, if
they are CTFE values.
2nd try at pull #2205
Replace 0 with NULL
Make implicit cast from long double to integer explicit
Merge pull request #2214 from yebblies/zeronull

[DDMD] Replace 0 with NULL
Merge pull request #2213 from yebblies/floatint

[DDMD] Make implicit cast from long double to integer explicit
Merge pull request #2212 from WalterBright/refactor_error_check2

2nd try at pull #2205
Merge pull request #2216 from donc/ctfe9634structconcat

9634 [CTFE] wrong code concatenating arrays of structs
Merge pull request #2217 from donc/refactorCtfeAssign

Refactor CTFE assign