Add `-defun'

[nbfalcon]

-defun is like -lambda, but with destructuring. To implement it, abstract the arglist and form generation code of -lambda into separate functions.

Additional benefits of the refactoring are:

• -lambda (and by extension, -defun) can now accept empty argument lists
• -lambda (and -defun) no longer rebinds symbols (fixing the TODO)

Various declaration forms should be handled properly in -defun.

Motivation: when this is merged, lsp-defun' can be made an alias for -defun`.

• Please note the use of intern instead of make-symbol. Using the latter instead caused test-failures.

• What git-commit-major-mode do you prefer? I assumed and used git-commit-elisp-text-mode here

• [x] add examples