Do not use the `fields` build() and create() arg

Remove all usages of the `field` argument. For example, this:

    SomeEntity().create(fields={'field': value})

Is now written as this:

    SomeEntity(field=value).create()

The `fields` argument to `build()` and `create()` should not be used, as it is
highly error-prone.
Merge pull request #1198 from Ichimonji10/dont-use-fields

Do not use the `fields` build() and create() arg