Before, if you did something like:
```
const hi1 = "hi";
const hi2 = hi1;
```
This would create the "hi" data twice in the built object.
But since the value is const we don't have to duplicate the
data, now we take advantage of this fact.
closes#336
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