isthmus: FieldSelectionConverter converts the container operand before the ROW and MAP branches decline #1409

FieldSelectionConverter.convert converts the container operand (topLevelConverter.apply(toDereference)) before the ROW and MAP branches decide whether they can handle the index, so a call they decline has already converted, and then thrown away, its whole operand. #1261 moved the decline for unsafe OFFSET/ORDINAL ahead of the conversion, but the other declines still come after it: a ROW subscript with a non-integer literal (r['q'], see #1385), a MAP subscript whose key isn't a fixed-char literal (m[1] on MAP<INTEGER, INTEGER>, see #1386), and containers of any other type.

// m : MAP<INTEGER, INTEGER>
ITEM(m, 1)  // converts m once, then: IllegalArgumentException: Unable to convert call ITEM(map<i32,i32>, i32).

Measured on main at af247e6. With the default converters this is wasted work, and an error from converting the operand surfaces instead of the decline. When a converter added after the built-ins (the ConverterProvider.Builder call-converter list) does claim the call, it converts the operand again, so nested declined subscripts convert their innermost operand 2^depth times. Deciding from the container type and the index literal before calling topLevelConverter.apply fixes all three.