extra::ebml: Make reader::Res public

Since reader::vuint_at() returns a result of type reader::Res it makes sense
to make it public.

Due to rust's current behavior of externally referenced private structures,
https://github.com/mozilla/rust/issues/10573, you could still use the result and
assign it to a variable if you let the compiler do the type assignment,
but you could not explicitly annotate a variable to hold a reader::Res.
extra::ebml: Add unit test for vuint_at()
ebml::extra: Optimize reader::vuint_at()

Use a lookup table, SHIFT_MASK_TABLE, that for every possible four
bit prefix holds the number of times the value should be right shifted and what
the right shifted value should be masked with. This way we can get rid of the
branches which in my testing gives approximately a 2x speedup.
fixup! ebml::extra: Optimize reader::vuint_at()
auto merge of #11498 : c-a/rust/optimize_vuint_at, r=alexcrichton

Use a lookup table, SHIFT_MASK_TABLE, that for every possible four
bit prefix holds the number of times the value should be right shifted and what
the right shifted value should be masked with. This way we can get rid of the
branches which in my testing gives approximately a 2x speedup.

Timings on Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz

-- Before --
running 5 tests
test ebml::tests::test_vuint_at ... ok
test ebml::bench::vuint_at_A_aligned          ... bench:       494 ns/iter (+/- 3)
test ebml::bench::vuint_at_A_unaligned        ... bench:       494 ns/iter (+/- 4)
test ebml::bench::vuint_at_D_aligned          ... bench:       467 ns/iter (+/- 5)
test ebml::bench::vuint_at_D_unaligned        ... bench:       467 ns/iter (+/- 5)

-- After --
running 5 tests
test ebml::tests::test_vuint_at ... ok
test ebml::bench::vuint_at_A_aligned ... bench: 181 ns/iter (+/- 2)
test ebml::bench::vuint_at_A_unaligned ... bench: 192 ns/iter (+/- 1)
test ebml::bench::vuint_at_D_aligned ... bench: 181 ns/iter (+/- 3)
test ebml::bench::vuint_at_D_unaligned ... bench: 197 ns/iter (+/- 6)