Make a distinction between repr() and toString()

[kyonifer]

In #78 it was proposed that we have two string representation methods for containers: toString() which presents a clear human-readable output like [1, 2, 3], and repr() which would produce a string containing a compileable code snippet that recreates the array, such as ndArrayOf(1, 2, 3). Something along these lines was also posed by @thomasnield, so I think it makes sense to go ahead and make two distinct string functions.