raco setup: reorganize command-line flags

Group flags and refine the descriptions and documentation.

Merge to v6.1
reference: clarify significance of order for submodules

Merge to v6.1
fix doc typo

Closes PR 14626

Merge to v6.1
compiler/zo-structs: document `undef-ok?` in `assign`

Closes PR 14622

Merge to v6.1
fix marshaling of #'(....  . ()) to bytecode
raco setup -c => disable everything else
optimizer: transform (if v x v) to (if v x #f)
optimizer: allow to move once_used to test position
optimizer: generalize moving expressions to single-value context

Gemeralize Gustavo's change so that immediately-used right-hand sides
can be moved into any position that (like the binding context) enforces
single-valuedness --- for arbitrary right-hand expressions.