Add suggestions for `std_instead_of_core`

[kpreid]

Description

If the std_instead_of_core lint (and its relatives std_instead_of_alloc and alloc_instead_of_core) generated suggested fixes, then it would be easy to bulk migrate imports using cargo clippy --fix, which would make it easier to explore adding no_std compatibility to a library.

Version

No response

Additional Labels

@rustbot label +C-enhancement

[tom-anders]

@rustbot claim