Adds and changes descriptions for mech parts. Adjusts w_class.

For some reason everything inheriting from mecha_parts had a w_class of 20, leaving a glitchy blank spot in the examine description. Changes to 5 so they're correctly classified as "huge" items in the examine text. They'll fit into a bag of holding as a result, but whatever.

Also adds and changes descriptions to just about all mech parts, for those who like to shift click absolutely everything.