added missing opcode FX1E
added missing opcode FX1E

* Forgot the FX1E opcode ; to insure all the opcodes are here all
opcodes comments now contain their name for easier identification.
Also improved the op_not_implemented! macro printing

* Tested more demo ROMs : most work, but some do not seem to do so.
Drawing seems buggy sometimes, as do the input handling, and some
opcodes must certainly still be wrongly implemented.

pierreyoda/rust-chip8