Restore 1.60 MSRV and fix build

[tarcieri]

Seems some changes I force pushed to #123 didn't wind up getting merged. In that PR, I noted that pub const fn new was MSRV breaking and got rid of the const but that didn't end up in develop.

I also encountered a build failure on develop since the legacy black_box function wasn't gated on the core_hint_black_box feature and thus clashed with core::hint::black_box when it was imported:

   Compiling subtle v2.6.0 (/Users/tarcieri/src/subtle)
error[E0255]: the name `black_box` is defined multiple times
   --> src/lib.rs:223:1
    |
100 | use core::hint::black_box;
    |     --------------------- previous import of the value `black_box` here
...
223 | fn black_box<T: Copy>(input: T) -> T {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `black_box` redefined here
    |
    = note: `black_box` must be defined only once in the value namespace of this module
help: you can use `as` to change the binding name of the import
    |
100 | use core::hint::black_box as other_black_box;
    |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

warning: unused import: `core::hint::black_box`
   --> src/lib.rs:100:5
    |
100 | use core::hint::black_box;
    |     ^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_imports)]` on by default

This is a breaking change since we shipped a const fn for BlackBox::new already, so I'd suggest releasing this as 2.6.1 and yanking 2.6.0 for being unintentionally MSRV breaking.