Check that keys of nested dictionaries in annotations are str

[zm711]

Fixes #1195.

Just adding a check for the key. Example from issue:

my_meta={(lambda a: None): 5}

Because the keys aren't checked this doesn't fail until writing. We should check to make sure the key is one of the allowable annotations.

We still have problems with writing some of the allowable annotations (see this issue), but adding this check at least moves in the right direction.

[apdavison]

I think we'd always assumed that annotation keys would be strings, since they represent names. I don't think anyone would be surprised if they got an error when using a lambda function as the name for an annotation.

If we're going to check this, then only str should be allowed for annotation keys and sub-keys, with a TypeError raised otherwise.

[zm711]

That makes things easier, so I'm happy to switch that :)