fields with leading underscore

[1st1]

Fields starting with underscore have a special meaning in Pydantic and cannot be supported out of the gate

Things to fix / take care of / ideas:

• Generator should issue a warning when it sees a field with leading underscore or a field that's not valid in Python for some reason
• GelModel machinery should mangle the name so that pydantic can have this field at all
• GelModel machinery should add an alias arg to FieldInfo with the original unmangled field name. That will make pydantic accept the original field from input.
• We can also potentially create computeds with demangled names that can write and read from mangled field. Test this.

[1st1]

type Spam{
  _foo: str
}

class Spam:
  p_foo: Property[str, '_foo']

or even

class Spam:
  p_foo: RenamedProperty[str, '_foo']

Instead of p we can have a long suffix or a long prefix (or some awkward and less likely to be invented by a human stuff like P_foo)