Given a field key and a meta_key, makes up a string that lets you
access that meta_key of the field.
<field_key "weight" "type"/>"weight_f_type"
The "meta_key" for the main value of a field is "value".
"value" is the default value for the meta_key paramter.
<field_key "weight" "value"/>"weight"
<field_key "weight"/>"weight"
If key is a string (the usual case) or if meta_key is not "value"
(also usual), field_key returns a string, even if
either or both of its arguments are not strings.
Before building the return value, field_key calls
to_cxs on both of its arguments and uses the results.
