Add names for base instructions

[Linda-Njau]

No description provided.

[ThinkOpenly]

Thanks! There is a lot of subjectivity here, and the source manual is sometimes inconsistent, so I get to do it my way! ;-)
In general:

• all lower case
• try to treat the mnemonic as an initialism, such that each letter turns directly into a word starting with that letter
• keep the primary action as the first word if there is a choice

For "sla", "shift left arithmetic" is preferred over "arithmetic left shift", for example.

[Linda-Njau]

Changed and squashed.

[Linda-Njau]

I've updated the names to lowercase as we discussed.