std: Remove implicit shrinking from hash_map.

Implements fn shrink_to_fit for HashMap.
std: Change the behavior of `reserve` for HashMap.

HashMap's `reserve` method now takes as an argument the *extra* space
to reserve.

[breaking-change]
Treat builtin bounds like all other kinds of trait matches. Introduce a simple hashset in the fulfillment context to catch cases where we register the exact same obligation twice. This helps prevent duplicate error reports but also handles the recursive obligations created by builtin bounds.
Correct various compile-fail tests. Most of the changes are because we
now don't print duplicate errors within one context, so I sometimes
had to break functions into two functions.
FIXME(#19481) -- workaround valgrind cleanup failure (but the code is nicer this way anyhow)
test: Ignore issue-19501 pretty for now

I don't have enough time to investigate this thoroughly, so for now let's get
the queue moving by ignoring this test.

cc #19501
auto merge of #19502 : alexcrichton/rust/issue-19501, r=sfackler

I don't have enough time to investigate this thoroughly, so for now let's get
the queue moving by ignoring this test.

cc #19501
auto merge of #18749 : nikomatsakis/rust/builtin-bounds-like-other-traits, r=pcwalton

Treat builtin bounds like all other kinds of trait matches. Introduce a simple hashset in the fulfillment context to catch cases where we register the exact same obligation twice. This helps prevent duplicate error reports but also handles the recursive obligations created by builtin bounds.

r? @pcwalton 
cc @FlaPer87
auto merge of #18770 : pczarn/rust/hash_map-explicit-shrinking, r=Gankro

Part of enforcing capacity-related conventions, for #18424, the collections reform.

Implements `fn shrink_to_fit` for HashMap.
The `reserve` method now takes as an argument the *extra* space to reserve.