Pass arguments by address in convert() templates

Pass arguments by address in convert() templates

The situation of the convert<>() template was a mess: the header
announced that template specializations passed strings by address (my
doing long ago), but the code would just define versions that upass
strings by value.

The solution is to pass all values by address. This works very well,
_except_ when the value is a string litteral.

Defining the templates thus requires a bit of work.

Spotted by Coverity scan.