Old:
```
while (condition; expression) {}
```
New:
```
while (condition) : (expression) {}
```
This is in preparation to allow nullable and
error union types as the condition. See #357
* comptime expression is a block expression as it should be
* fix var args when number of args passed is 0
* implement const value equality for structs
* fix indent when rendering container decl AST
* IR: prevent duplicate generation of code when it is partially
compile-time evaluated
* implement compile time struct field pointer evaluation
* fix compile time evaluation of slicing
Andrew Kelley