This extends the existing Rust code to handle conditional
replacement. The replacement of field names and filters to text
remains in Python, so that add-ons can still define their own
field modifiers.
The code is currently running the old Pystache rendering and the
new implementation in parallel, and will print a message to the
console if they don't match. If you notice any problems, please
let me know.
In the process of factoring out the field filtering, the "extra"
and "fullname" args are just passed in as a blank string now.
Extra was functionality that allowed a field modifier to be defined
as "filtername(arg1,arg2):field", and fullname was the name of the
field including any provided field modifiers. From grepping through
the add-ons on AnkiWeb, neither appears to have been used.
