Add Order as const generic

[vinaychandra]

This PR allows to switch from the default 32 level heap to a generic heap that could be of any level.

This is a breaking change because #![feature(const_generics_defaults)] is incomplete.

Resolves #9

[jiegec]

Great idea! But to maintain compatibility, can we:

1. rename Heap to another name
2. alias Heap as 32-ordered heap
3. or should we wait for const_generics_defaults?

[vinaychandra]

I can update Heap to OrderedHeap and then alias Heap to OrderedHeap<32>. const_generics_defaults can take a long time.
But again, the current change also doesn't work in stable because const generics is stablized only in nightly. Is that acceptable?

[jiegec]

I can update Heap to OrderedHeap and then alias Heap to OrderedHeap<32>. const_generics_defaults can take a long time.
But again, the current change also doesn't work in stable because const generics is stablized only in nightly. Is that acceptable?

It seems not far: https://blog.rust-lang.org/2021/02/26/const-generics-mvp-beta.html

[jiegec]

Here it is: https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html. Let's try new stable on this pr.

Let's wait for CI to bump to 1.51.0.