Tracking issue for the next breaking release

[newpavlov]

Potential changes:

• Migrate to extern_item_impls. See Tracking issue for migration to extern_item_impls #796.
• Remove free-standing functions in favor of SysRng. This would allow to simplify the implementation a bit, but would make rand_core a required dependency.
• Remove the file fallback from the default Linux backend.
• Use randomness API provided by std for targets with available std instead of re-implementing platform support in getrandom (see Tracking Issue for secure random data generation in std rust-lang/rust#130703)