Improve Reko disassembly speed

[AnErrupTion]

This PR drastically improves the disassembly speed when using Reko (a.k.a. the -output-asm CLI argument). The main problem was that the MOSA Disassembler class would:

• Read every disassembled instruction from the list created by the Reko library
• Create a whole new list with a few modifications in the string representation
• Read every instruction from that newly created list, the most common scenario being to write them into a file

This is indeed much slower, so this PR fixes it by simply having a DecodeNext() method in the Disassembler class, which makes it act like an enumerator.

[charsleysa]

LGTM