release 1.5 beta

• fixed loop labeling problem that resulted in uncompilable assembly code
• stackvm can now intercept certain system/rom calls
• stackvm endless loop bug fixed
• strlen() and sqrt16() added
• improved some compiler error messages
• for loops can now be over an iterable literal (such as a string or array) without explicitly having to use a hold variable
• irq handler now saves zero page scratch registers