Implement support for named fields, i.e.  where one field is defined
in terms of another, rather than directly in terms of bits extracted
from the instruction.

The new method referenced_fields() on all the Field classes returns a
list of fields that this field references.  This just passes through,
except for the new NamedField class.

We can then use referenced_fields() to:
 * construct a list of 'dangling references' for a format or
   pattern, which is the fields that the format/pattern uses but
   doesn't define itself
 * do a topological sort, so that we output "field = value"
   assignments in an order that means that we assign a field before
   we reference it in a subsequent assignment
 * check when we output the code for a pattern whether we need to
   fill in the format fields before or after the pattern fields, and
   do other error checking

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230523120447.728365-6-peter.maydell@linaro.org>