peer type resolution for optional slice and single-pointer-to-array

[andrewrk]

Extracted from #4752.

As an example, try removing this @as cast here:

zig/lib/std/net.zig

Line 823 in dc04e97

break :blk @as([]u8, &line_buf); // TODO the cast should not be necessary

It gives:

lib/std/net.zig:819:60: error: incompatible types: '*[512]u8' and '?[]u8'

The first step to solving this is a self-contained reduced behavior test case.