Let RngCore: TryRngCore<Error = Infallible>

[dhardy]

• Added a CHANGELOG.md entry

Summary

Revise the core traits according to @newpavlov's suggestion and similar to #41 but keeping the infallible methods.

No trait renames are done in this PR. Also not sure if we need any.

Details

• TryRngCore is implemented for any R: DerefMut where Target: TryRngCore.
• RngCore: TryRngCore<Error = Infallible> with automatic implementation over TryRngCore
• UnwrapMut is removed
• CryptoRng = RngCore + TryCryptoRng or as close as possible without sum traits; it is kept since I believe some people need dyn CryptoRng
• utils::{next_u64_via_u32, next_word_via_fill} now use closures instead of R: RngCore

Questions

Should utils fns support potentially-fallible uses (try_next_u64_via_u32 takes FnMut() -> Result<u32; E> for generic E)? I think unnecessary.

[dhardy]

Updated. Some things need further discussion (see above).