Minify CellType

[adriandelgado]

I changed some fields' types of the CellType enum from String to Box<str> or Rc<str>. Result:

	Before	After
Size (Bytes)	80	56
Alignment	8	6

This improves cache locality and allows the compiler to better optimize its use.

I also refactored the prepare_formula function because I needed it to output a Box<str> instead of a String.

Why Rc<str>?

To improve the performance of the shared_string_index function. This is a hot function that needs to clone strings some of the time. By using Rc<str> the clones become really cheap.

[jmcnamara]

Looks good. Merged. Thanks.