std: micro-optimize Vec constructors and add benchmarks

Generally speaking, inlining doesn't really help out with
constructing vectors, except for when we construct a zero-sized
vector. This patch allows llvm to optimize this case away in
a lot of cases, which shaves off 4-8ns. It's not much, but it
might help in some inner loop somewhere.

before:

running 12 tests
test bench_extend_0          ... bench:       123 ns/iter (+/- 6)
test bench_extend_5          ... bench:       323 ns/iter (+/- 11)
test bench_from_fn_0         ... bench:         7 ns/iter (+/- 0)
test bench_from_fn_5         ... bench:        49 ns/iter (+/- 6)
test bench_from_iter_0       ... bench:        11 ns/iter (+/- 0)
test bench_from_iter_5       ... bench:       176 ns/iter (+/- 11)
test bench_from_slice_0      ... bench:         8 ns/iter (+/- 1)
test bench_from_slice_5      ... bench:        73 ns/iter (+/- 5)
test bench_new               ... bench:         0 ns/iter (+/- 0)
test bench_with_capacity_0   ... bench:         6 ns/iter (+/- 1)
test bench_with_capacity_100 ... bench:        41 ns/iter (+/- 3)
test bench_with_capacity_5   ... bench:        40 ns/iter (+/- 2)

after:

test bench_extend_0          ... bench:       123 ns/iter (+/- 7)
test bench_extend_5          ... bench:       339 ns/iter (+/- 27)
test bench_from_fn_0         ... bench:         7 ns/iter (+/- 0)
test bench_from_fn_5         ... bench:        54 ns/iter (+/- 4)
test bench_from_iter_0       ... bench:        11 ns/iter (+/- 1)
test bench_from_iter_5       ... bench:       182 ns/iter (+/- 16)
test bench_from_slice_0      ... bench:         4 ns/iter (+/- 0)
test bench_from_slice_5      ... bench:        62 ns/iter (+/- 3)
test bench_new               ... bench:         0 ns/iter (+/- 0)
test bench_with_capacity_0   ... bench:         0 ns/iter (+/- 0)
test bench_with_capacity_100 ... bench:        41 ns/iter (+/- 1)
test bench_with_capacity_5   ... bench:        41 ns/iter (+/- 3)
serialize: Simplify the json docs
std: inline many of the Writer/Reader methods

This allows llvm to optimize away much of the overhead from using
the MemReader/MemWriters. My benchmarks showed it to shave 15% off
of my in progress serialization/json encoding.
collections: fix running `make check-stage1-collections`
auto merge of #15068 : erickt/rust/mem-inline, r=pcwalton

This is a couple micro-optimizations I've been sitting on for a while. This inlines a couple functions that are important to the `std::io::mem`. Ultimately, this results in about a 15% performance increase in some micro-benchmarks for my [libserialize](https://github.com/erickt/rust-serde) rewrite.