This updated contrib/capnproto to a newer version that fixes problems with
unaligned access to message frames.

It also adds support for parsing Struct types as Tuple (named or unnamed),
and Nested array types.

The `struct X { a @0 :UInt64; b @1 :Text }` in Cap'nProto is equivalent to
`x Tuple(a UInt64, b String)` in ClickHouse.

Arrays of Struct types such as `y List(X)` are equivalent to `y Nested(a UInt64, b String)`.