recreator: added support for string and sub-string assignments

the assign string recreate function handles the assign string, assign
left, assign mid 2 and 3, assign right, assign keep string, assign keep
left, assign keep mid 2 and 3, and assign keep right codes; the pointer
to this function was added to the table entries for these codes; and
second associated codes was given to all the assign sub-string codes to
associated back to the origin string functions (needed to get the number
of arguments of the sub-string function)

added new separator member to the recreator class along with access
functions to keep track of the current separator between each of the
assign keep codes in a multiple sub-string assignment statement

translator tests 1 to 5 (various assignment tests) all produce the
correct recreator output with the exception of test 5 (command tests),
which contains a single PRINT statement that is not yet implemented