Change two-bit stack map entries to encode:
0 = dead
1 = scalar
2 = pointer
3 = multiword

If multiword, the two-bit entry for the following word encodes:
0 = string
1 = slice
2 = iface
3 = eface

That way, during stack scanning we can check if a string
is zero length or a slice has zero capacity.  We can avoid
following the contained pointer in those cases.  It is safe
to do so because it can never be dereferenced, and it is
desirable to do so because it may cause false retention
of the following block in memory.

Slice feature turned off until issue 7564 is fixed.

Update #7549

LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/76380043