Add decorator syntax for method fields?

Nobody has claimed this yet.

The current syntax for method fields is somewhat awkward, especially around naming:

```python
class MySchema(Schema):
is_valid = fields.Method('get_is_valid')

def get_is_valid(obj):
return something_something_something(obj)
```

At a slight cost to consistency, we could make this more ergonomic with syntax like:

```python
class MySchema(Schema):
@fields.method
def is_valid(obj):
return something_something_something(obj)
```

And/or:

```python
class MySchema(Schema):
@fields.method(dump_to='foo')
def is_valid(obj):
return something_something_something(obj)
```

Does this sound reasonable?

If so, is the naming okay? And should `@fields.method` be allowed as shorthand when not passing any other arguments in setting up the field, or should it always be `@fields.method()`?

The issue names no files or tests. Start by locating the existing fields.Method implementation and the Schema field-declaration handling, then review the proposed decorator forms and their naming options. Done would require an agreed API design plus corresponding implementation and test coverage.

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