Allow patching of StringIdx values

[gatecat]

At the moment, patching of StringIdx values fails. Consider the following example:

parameters.yaml:

cells:
  - cellType: FOO

running

python -mfpga_interchange.patch  --schema_dir ~/fpga-interchange-schema/interchange --patch_path parameterDefs --schema device --patch_format yaml LIFCL-17.device parameters.yaml LIFCL-17-patched.device

fails with

Traceback (most recent call last):
  File "/usr/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/david/python-fpga-interchange/fpga_interchange/patch.py", line 72, in <module>
    main()
  File "/home/david/python-fpga-interchange/fpga_interchange/patch.py", line 65, in main
    patch_capnp(message, patch_path, args.patch_format, f)
  File "/home/david/python-fpga-interchange/fpga_interchange/patch.py", line 37, in patch_capnp
    read_format_to_message(message_to_populate, patch_format, in_f)
  File "/home/david/python-fpga-interchange/fpga_interchange/convert.py", line 83, in read_format_to_message
    from_rapidyaml(message, yaml_tree)
  File "/home/david/python-fpga-interchange/fpga_interchange/rapidyaml_support.py", line 393, in from_rapidyaml
    from_reader(message, data, RapidYamlReader)
  File "/home/david/python-fpga-interchange/fpga_interchange/converters.py", line 458, in from_reader
    from_reader(
  File "/home/david/python-fpga-interchange/fpga_interchange/converters.py", line 479, in from_reader
    value = reader.read_scalar(field_which, value)
  File "/home/david/python-fpga-interchange/fpga_interchange/rapidyaml_support.py", line 335, in read_scalar
    return int(field_data)
ValueError: invalid literal for int() with base 10: 'FOO'

As there is no special casing for indexed strings, it is expecting an index into the string pool rather than a string (which isn't feasibly human-writeable). These will need to be handled by looking into, and updating if needed, the list of strings. This is also a prerequisite to fixing chipsalliance/fpga-interchange-schema#46, hitherto this has been worked around in the schema by using Text instead of StringIdx where the patching tool needs to be used.