Integer indexes

[karliss]

Array indexes were sometimes integers, sometimes floats causing Append to fail on simple code.

array a = [1]
Append(a, 2)

More ways of causing problems can be found in tests.

Changes:

• Add float casts when creating array indexes to eleminitate most sources of integers
• Handle integers in Append, just in case some integers remain
• Throw an Error from Append in case of index conflict, the old code threw an unhandled exception in such case

[eriksvedang]

Very nice! I'll look through and merge this on Monday.