Avoid double indirection for the "self" arg in methods

[dotdash]

Currently we pass all "self" arguments by reference, for the pointer
variants this means that we end up with double indirection which causes
a unnecessary performance hit.

The fix itself is pretty straight-forward and just means that "self"
needs to be handled like any other argument, except for by-value "self"
which still needs to be passed by reference. This is because
non-pointer types can't just be stuffed into the environment slot which
is used to pass "self".

What made things tricky is that there was also a bug in the typechecker
where the method map entries are created. For type impls, that stored
the base type instead of the actual self-type in the method map, e.g.
Foo instead of &Foo for &self. That worked with pass-by-reference, but
fails with pass-by-value which needs the real type.

Code that makes use of methods seems to be about 10% faster with this
change. Also, build times are reduced by about 4%.

Fixes#4355, #4402, #5280, #4406 and #7285

[graydon]

Didn't someone just land the exact opposite of this change?

[dotdash]

You're probably referring to #7410 by @luqmana which made "self" (without any sigil) be passed by reference. That's the one I keep as "by reference", as I mentioned in the commit/PR message.

Though I did revert a few changes from that commit that were required to tell the self and &@~self apart.

Edit: @luqmana did that, not @msullivan.

[dotdash]

Can't reproduce this failure locally, neither with x86_64 nor with a i686 cross compile. Interestingly, bors didn't add a link to the log of the test run that actually failed, which is http://buildbot.rust-lang.org/builders/auto-linux-32-opt/builds/174/steps/test/logs/stdio. Any ideas?