Previously, an error was returned during unmarshal when a wiretype
was encountered that did not match the expected wiretype.

In order to match the behavior of the C++ and Python implementations,
we no longer return an error and instead store the bad wire fragment
as an unknown field (or skip them if unknown field preservation is disabled).

The generator still produces code that references ErrInternalBadWireType
for unmarshal logic for oneof fields.
However, the current proto package does not use the generated unmarshalers
for oneofs, so their existence has no bearing on unmarshal semantics.

Cleaning up the generator to stop producing these is future work.

…tocolbuffers#565)

A prior changed (see protocolbuffers#511) allows Unmarshal to treat fields with
unknown wire types with known tags as unknown fields.
When doing so, we must be careful not to set the required field bit.

For example, suppose we have:
	message Foo { require fixed32 my_field = 1; }

Then parsing a message with a field of type varint and tag 1 should
not satisfy the required that Foo.my_field be set.

…num value options (no wrapper), fixes protocolbuffers#510; expose zero on LongBits, fixes protocolbuffers#508