Remove redundant bounds

[steffahn]

There were 3 trait implementations that placed redundant AsRef/AsMut bounds on <R as BlockRngCore>::Results which were already implied by the bounds in the BlockRngCore trait definition.

Noticed those because they show up in the documentation, e.g. here.

Also fix 2 warnings (one clippy warning, one rustc warning).

[dhardy]

Sorry for the delay. Looks fine if it passes CI.

[dhardy]

Which it doesn't (this is using Rust 1.36.0):

error[E0277]: the trait bound `<R as block::BlockRngCore>::Results: core::convert::AsRef<[u32]>` is not satisfied
   --> rand_core/src/block.rs:181:1
    |
181 | / impl<R: BlockRngCore<Item = u32>> RngCore for BlockRng<R> {
182 | |     #[inline]
183 | |     fn next_u32(&mut self) -> u32 {
184 | |         if self.index >= self.results.as_ref().len() {
...   |
237 | |     }
238 | | }
    | |_^ the trait `core::convert::AsRef<[u32]>` is not implemented for `<R as block::BlockRngCore>::Results`
    |
    = help: consider adding a `where <R as block::BlockRngCore>::Results: core::convert::AsRef<[u32]>` bound
note: required by `block::BlockRngCore`
   --> rand_core/src/block.rs:68:1
    |
68  | pub trait BlockRngCore {
    | ^^^^^^^^^^^^^^^^^^^^^^

[steffahn]

Oh, so Rust used to not support this? Interesting...

[steffahn]

I'll investigate later what changed when, etc. Keeping these bounds really is only an issue of documentation after all, so it's not too bad. If we need to keep them, I might change this PR so that comments are added in the source code on these bounds, pointing out they're redundant but used to be needed and are kept for MSRV support.

[dhardy]

1.36.0 is 2.5 years old by now; when we asked there was generally little interest in support beyond six months old, so an MSRV bump is not out of the question (even if this is only a minor thing).

[steffahn]

The relevant fix in rustc was apparently rust-lang/rust#73905. The code without the extra bound (i.e. the code in this PR) works from Rust 1.49 (released December 2020).

[vks]

This needs an update to the changelog and a bump to Rust 1.51 (see #1165).

[dhardy]

@steffahn can you rebase? The MSRV was increased in #1246 already.

[steffahn]

Rebased