`Expansion::advance()` can increase its offset beyond the size,
so I don't think this assumption was valid in the first place;
`BufferedContent::advance()` should be able to as well.
Explicit {interpolation} can still achieve this, but
to match DEF, REDEF, and PURGE, these new directives that
define symbols do not expand string EQUS.
Fixes#669Closes#368Closes#624
Deprecate PRINTT, PRINTV, PRINTI, and PRINTF
Default STRFMT("%f") to 5 fractional digits like "{f:}"
Any use of string formatting will share this default
