an implementation of varint reading/writing
first step towards using varint in serialization.

serialize arrays of integers with varints
as well as regular integers
teach the 6model reprs about varints

this currently segfaults upon
deserialization while building nqp.
turns out this is actually needed
special treatment for the last byte

we need that to reach full 64 bits, otherwise
we are short exactly one bit.

and in the 9th bit we already know we're going
to stop reading anyway.
varint128 is a bogus name, since it's 9bytes max.
Merge branch 'varint'

this gives us variable-sized integers for serialization
blobs. There's only very, very few integers that even
need 5 bytes with our varint scheme, so we save loads of
space with this!