libcollections: remove `init_to_vec`

The `init_to_vec` function in `collections::bitv` was exposed as an
inherent method, but appears to just be a helper function for the
`to_vec` method. This patch inlines the definition, removing
`init_to_vec` from the public API.

[breaking-change]
test: index shards at 1, not 0

This has no tests because it's near impossible to test -- since TestFn uses
`proc`s, they can not be cloned or tested for equality. The only way to really
test this is making sure that for a given number of shards `a`, sharding from
1 to `a` yields the complete set of tests. But `filter_tests` takes its vector
by value and `proc`s cannot be compared.

[breaking-change]

Closes #10898
auto merge of #14286 : cmr/rust/shard-benches, r=alexcrichton

This has no tests because it's near impossible to test -- since TestFn uses
`proc`s, they can not be cloned or tested for equality. The only way to really
test this is making sure that for a given number of shards `a`, sharding from
1 to `a` yields the complete set of tests. But `filter_tests` takes its vector
by value and `proc`s cannot be compared.

[breaking-change]

Closes #10898
auto merge of #14295 : aturon/rust/hide-init_to_vec, r=alexcrichton

The `init_to_vec` function in `collections::bitv` was exposed as an
inherent method, but appears to just be a helper function for the
`to_vec` method. This patch inlines the definition, removing
`init_to_vec` from the public API.

[breaking-change]