Suggestion: merge `echo-element` and `echo-segment` into `echo-style`

[balupton]

Work has started on this, including the following:

• [x] enable choose to be themeable
• [x] support nocolor variants for segments and elements
• [x] rework targets for better performance now that there is native nocolor support
• [x] Simplify and improve alias handling, which will improve handling of disable styles

Remaining todos:

• [x] continuation styling is broken