Partially decouple NTA proxy objects

Proxy objects are used to locate NTA children from parameterized NTAs
during inherited attribute evaluation. Previously the proxy object was a
list, and the list could be used to iterate over evaluated parameterized
NTA values.

Proxy objects are now plain ASTNode instances, and when not using
rewrites or using circular NTA rewrites they do not contain references
to the child nodes using them as proxy parents.

Null references are no longer stored in the proxy objects in any
configuration.

This change also renames the proxy objects from attrname_list to
attrname_proxy.

see #112 (bitbucket)