Version 2.5.3 - DD, DF, DFF, DFZXS

A new directives for defining numbers, like DB and DW:

• DD for defining double words (4 bytes)
• DF for defining floating point number (IEEE-754), 32bit
• DFF for defining double precision FP number (64bit)
• DFZXS for defining floating point number in the ZX Spectrum style (warning: 0.5 error is fixed, so no 7f 7f ff ff ff)