Assigns a value to a variable.
|First call GetVar. This returns the address of the variable's value in DE, allocating space for it if it doesn't already exist.|
|Check that '=' follows the variable name.|
Assigns the result of the BASIC expression at HL to the variable pointed to by DE.
|Preserve variable address on stack and get the variable value into FACCUM.|
|Swap (SP) and HL, so HL now holds the address of the variable's value and the program ptr is on the stack.|
|Store variable address in PROG_PTR_TEMP, don't know why.|
|Store FACCUM (the expression result) in the variable and get the variable value address into DE.|
|Restore program ptr to HL and return.|