Eliminate the trailing newlines and blank spaces (the code called them
"a stupid artifact").

Include the name of the defining interface (so the user can easily look up
any requirements on the attribute) and, for methods, the expected
signature (no more guessing about how many arguments are required!).

This is implemented by giving Attribute and Method useful reprs and strs.
Previously, they just had the defaults.

Fixes #170

On PyPy2, they are ignored (like on CPython), but on PyPy3 they can
actually be validated.

Fixes #118

Fixes #153

The items that went in each ``__all__`` are based on what is
documented:

 $ rg --no-filename 'import' docs/ -trst | tr -s "[:blank:]" | sort | uniq | grep zope
 >>> from pprint import pprint
 >>> from zope.interface import *
 >>> from zope.interface import Interface
 >>> from zope.interface import Interface, Attribute, implementer
 >>> from zope.interface import alsoProvides
 >>> from zope.interface import classImplements
 >>> from zope.interface import classImplementsOnly
 >>> from zope.interface import directlyProvidedBy
 >>> from zope.interface import directlyProvides
 >>> from zope.interface import implementedBy
 >>> from zope.interface import implementer
 >>> from zope.interface import implementer_only
 >>> from zope.interface import noLongerProvides
 >>> from zope.interface import providedBy
 >>> from zope.interface import provider
 >>> from zope.interface.adapter import AdapterRegistry
 >>> from zope.interface.declarations import Declaration
 >>> from zope.interface.declarations import InstanceDeclarations
 >>> from zope.interface.declarations import ProvidesClass
 >>> from zope.interface.declarations import named
 >>> from zope.interface.exceptions import BrokenImplementation
 >>> from zope.interface.exceptions import Invalid
 >>> from zope.interface.interface import Specification
 >>> from zope.interface.interface import adapter_hooks
 >>> from zope.interface.verify import verifyObject
 >>> import gc
 >>> import zope.interface

And also some personal judgement about what the public API is that I'm
more than happy to have reviewed.

Turns out they can be called in some very strange error cases. See #162 and #163 for details.

This should fix #162 (at least the provided test case, five.intid, passes now).

It also does enough work on #163 that (a) the test can be written and run in pure-python mode, which was needed to debug it and (b) five.intid runs in pure-python mode (well, with a bunch of other small hacks to Acquisition, ExtensionClass, DocumentTemplate and AccessControl), but I won't claim that it fully fixes #163. For one thing, there are no specific tests. For another, I see more such differences.