fix: ensure parsing schema metadata isn't ambiguous

[alecthomas]

An example of the ambiguity is on fields. Given an alias:

data Data {
  field Field alias json "foo"
}

This effectively prevents the next field from being named "alias":

data Data {
  field Field
  alias json "foo"
}

Metadata is now disambiguated by requiring a + before every piece of metadata:

data Data {
  field Field +alias json "foo"
}

(also converted field aliases to metadata)