Allow patching of lists

Consider trying to patch the exception map, for example to test SymbiFlow/fpga-interchange-schema#42. Aside from the `StringIdx` issue in #81, patching something that is a list rather than a struct fails even earlier.

exceptions.yaml:

```
- primName: FOO
macroName: BAR
```

run
```
python -mfpga_interchange.patch --schema_dir ~/fpga-interchange-schema/interchange --patch_path exceptionMap --schema device --patch_format yaml LIFCL-17.device exceptions.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
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 30, in patch_capnp
message_to_populate = message_to_populate.init(patch_path[-1])
File "capnp/lib/capnp.pyx", line 1272, in capnp.lib.capnp._DynamicStructBuilder.init
File "capnp/lib/capnp.pyx", line 1288, in capnp.lib.capnp._DynamicStructBuilder.init
capnp.lib.capnp.KjException: capnp/dynamic.c++:693: failed: init() without a size is only valid for struct and object fields.
stack: 7f32523e6be1 7f3252734173 7f32526aca3b 7f325321a252 7f325320071c 7f32531fc461 7f3253208269 7f32531f77b6 7f3253208269 7f32531f77b6 7f32531f628c 7f32531f5c50 7f32532b7b12 7f32532beedc 7f3253208c40 7f32531f77b6 7f32531f628c 7f325320840d 7f32531f77b6 7f32531f628c 7f325320840d 7f32532e2f67 7f32532d9a91 7f32532aaba8 7f3252f2d151 55bda411d04d
```