Value is already defined as static earlier in the file
Explain what c2s means
Replace public usage of c2s with a wrapper function

Adds clarity because of the more informative name. Also allows the
use of a precondition to prevent out-of-bounds array access.

A short function name is not possible because it will conflict
with a similar function in piece.d. The reason is Coordinate and
Piece are both simply aliases to the same type so the following
function signatures would be the same and the compiler would
get confused

    string toString(Coordinate sq)
    string toString(Piece p)
Merge pull request #97 from joichiro/coordinatestatic

Replace the public usage of c2s with a function