package-manager documentation edits
scribble: add 'roman as a built-in element style
scribble/bnf: use 'roman for all BNF metasyntax

This change solves a long-standing problem with using @nonterm{}
inside @tt{} or other constructors that change the font, where
the font would be different (and uglier) than uses outside of font
changes.
allow hash-table chaperones to support efficient `hash-clear[!]`
raco pkg: improve package-source checking and error reporting