Macros for defining symex commands

[countvajhula]

Summary of Changes

This adds a couple of macros for defining symex commands. These macros handle the boilerplate of reindenting after execution, etc. and minimize the use of advice for such things, and could also implicitly include other common functionality like evil-repeat (dot .) support which currently requires a separate registration step. In general these macros represent a better abstraction of a command than using functions with ad hoc and inconsistently used set up and tear down code, and should allow us to integrate more easily with other tools like tree-edit. See #73 for a full description.

Bugs remaining:

• [x] paste-before with multiple lines yanked leaves the original line untidy
• [ ] replace (s) in string leaves cursor inside the quotes
• [x] S delimiter with { and } work the same and { doesn't do what it should
• [ ] shift backward/forward most doesn't move the body of the function to defun (probably related to indent level and maybe selection? verify whether it happens on the main branch)
• [ ] capture backward captures beyond current level at head
• [x] when on the last expression, & joins to previous instead of doing nothing
• [x] append newline inserts extra leading whitespace when indented (verify behavior on main) - also note insert newline doesn't do this
• [ ] J within an expression with next expression on same line joins next line instead of NOOP
• [ ] M-J collapse with comments present mangles structure
• [x] C-; evals the next symex not the current one (except if only one)

Public Domain Dedication

• [x] In contributing, I relinquish any copyright claims on my contribution and freely release it into the public domain in the simple hope that it will provide value.

(Why: The freely released, copyright-free work in this repository represents an investment in a better way of doing things called attribution-based economics. Attribution-based economics is based on the simple idea that we gain more by giving more, not by holding on to things that, truly, we could only create because we, in our turn, received from others. As it turns out, an economic system based on attribution -- where those who give more are more empowered -- is significantly more efficient than capitalism while also being stable and fair (unlike capitalism, on both counts), giving it transformative power to elevate the human condition and address the problems that face us today along with a host of others that have been intractable since the beginning. You can help make this a reality by releasing your work in the same way -- freely into the public domain in the simple hope of providing value. Learn more about attribution-based economics at drym.org, tell your friends, do your part.)

[countvajhula]

This PR was accumulating improvements beyond its scope, so I'm breaking this up into separate PRs. This one just introduces the new macros and rewrites existing commands using them. Although I'm going to go ahead and merge this, it will be reviewed as part of final review on the integration branch before being merged into the main branch. Of course, feedback is welcome at any time for anyone who has time to give this a look.