Trivial clean up for test/runnable/testbounds.d
partial fix Issue 3652 - Allow explicit and implicit casting of dynamic array slices of known size to static array

If and only if both side of slice boundaries can be constant-folded, the conversion will succeed.

Limitation:
- Does not run CTFE.
- There is no deformation of formula.
Remove the commented out ctfe stuff and the error gagging
Add `Type *SliceExp::toStaticArrayType()`
Merge pull request #1705 from 9rnsr/ct_slice

partial fix Issue 3652 - Allow explicit and implicit casting of dynamic array slices of known size to static array