In order to add these, we need to be able to find references
to such types that already exist in the binary. To do that, introduce
a new linker section holding a list of the types corresponding to
arrays, chans, maps, and slices.

To offset the storage cost of this list, and to simplify the code,
remove the interface{} header from the representation of a
runtime type. It was used in early versions of the code but was
made obsolete by the kind field: a switch on kind is more efficient
than a type switch.

In the godoc binary, removing the interface{} header cuts two
words from each of about 10,000 types. Adding back the list of pointers
to array, chan, map, and slice types reintroduces one word for
each of about 500 types. On a 64-bit machine, then, this CL *removes*
a net 156 kB of read-only data from the binary.

This CL does not include the needed support for precise garbage
collection. I have created issue 4375 to track that.

This CL also does not set the 'algorithm' - specifically the equality
and copy functions - for a new array correctly, so I have unexported
ArrayOf for now. That is also part of issue 4375.

Fixes #2339.

R=r, remyoudompheng, mirtchovski, iant
CC=golang-dev
https://golang.org/cl/6572043