Define and enforce MSRV

[kvark]

Looks like the crate depends on fairly fresh compiler features:

error[E0277]: arrays only have std trait implementations for lengths 0..=32
--> /Users/dmalyshau/.cargo/git/checkouts/gpu-alloc-b89b27c43ff2846f/0bdf9ea/gpu-alloc/src/usage.rs:51:5
|
51 | usages: [MemoryForOneUsage; 64],
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait std::array::LengthAtMost32 is not implemented for [usage::MemoryForOneUsage; 64]
|
= note: required because of the requirements on the impl of std::fmt::Debug for [usage::MemoryForOneUsage; 64]
= note: required because of the requirements on the impl of std::fmt::Debug for &[usage::MemoryForOneUsage; 64]
= note: required for the cast to the object type dyn std::fmt::Debug
= note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info

[zakarumych]

MSRV is 1.40
And 1.46 for gpu-alloc-erupt