Support array-style syntax for setting stack arena size per-callsite

[tnballo]

The current method for setting stack arena size via the SG_MAX_STACK_ELEMS environment variable has the advantage of keeping APIs compatible with BTreeMap/BTreeSet. For example, a new map can be declared in the usual way:

let map: SGMap<usize, usize> = SGMap::new();
assert_eq!(map.capacity(), 1024); // Assuming `SG_MAX_STACK_ELEMS == 1024` (default)

But this has the disadvantage of being a fixed, global setting. It'd be awesome if we could additionally support the array-style type declaration syntax used by crates like smallvec and tinyvec. This would allow unique stack arena sizes for each callsite, like so:

let default_map: SGMap<usize, usize> = SGMap::new();
assert_eq!(default_map.capacity(), 1024); // Assuming `SG_MAX_STACK_ELEMS == 1024` (default)

let little_map: SGMap<[(usize, usize); 256]> = SGMap::new();
assert_eq!(little_map.capacity(), 256);

let big_map: SGMap<[(usize, usize); 8192]> = SGMap::new();
assert_eq!(big_map.capacity(), 8192);

Making this change will require a little refactoring, the types in src/tree/types.rs would likely have to become associated with the SGTree struct instead of being global?

[tnballo]

Or a with_capacity constructor - potentially more ergonomic to use and cleaner to implement. Ex:

let little_map: SGMap<usize, usize> = SGMap::with_capacity(256);
assert_eq!(little_map.capacity(), 256);

[tnballo]

Branch v2.0.0 (release WIP) addresses this with const generics like so:

use core::mem::size_of_val;
use scapegoat::SGMap;

let small_map: SGMap<u64, u64, 100> = SGMap::new(); // 100 item capacity
let big_map: SGMap<u64, u64, 2_048> = SGMap::new(); // 2,048 item capacity

#[cfg(target_pointer_width = "64")]
{
    assert_eq!(size_of_val(&small_map), 2_696); // 2.7 KB
    assert_eq!(size_of_val(&big_map), 53_344);  // 53.3 KB
}

A bit of experimentation and research led to this design being the best solution!