Find a way to properly allocate the size of String variables, instead of using the theoretical maximum of 64 words (256 bytes)

[supercom32]

Issue identified in Dragon Basic as of commit ID 901c0e5.

In previous versions of Dragon Basic, Strings were allocated one word (4 bytes). From Uli's comments, there doesn't appear to be any way to specify the maximum size of a string variable. As a compromise, a patch was submitted so that we allocate the maximum of 64 words (256 bytes) instead. While this strategy indeed prevents data from being corrupted, it also wastes a bunch of memory as well. Since memory on the GBA is at a premium, we should probably try to find away to properly handle this in the future.