ledger-mode: Add namespace prefix to highlight-overlay

make-variable-buffer-local is a top-level form: this commit moves that
call from ledger-mode to next to the variable's declaration.
ledger-mode: Remove duplication in ledger-set-{year,month}
ledger-mode: Add missing argument to ledger-delete-current-transaction
ledger-mode: Fix a number of byte-compilation warnings
Don't default fill-column to 8 in Emacs
ledger-mode: Also bind g to ledger-report-redo, as is standard elsewhere