Add support for custom allocators by the allocator-api2 crate.

This commit adds allocator-api2 (a drop-in replacement for the
`Allocator` trait on stable) as a dependency. The underlying `Vec` is
replaced by `allocator_api2::vec::Vec`, and the type signiture of
`VecMap<T>` is changed to `VecMap<T, A: Allocator = Global>`.

It adds the methods `new_in()` and `with_capacity_in()` which creates a
`VecMap` with a custom allocator. The old `new()` and `with_capacity()`
methods are not removed and defaults to the default global allocator.

I think that this commit is backwards compatable.

Cargo.toml

@@ -39,8 +39,10 @@ readme = "README.md"
 exclude = ["/.travis.yml", "/deploy-docs.sh"]
 
 [features]
+serde = ["dep:serde", "allocator-api2/serde"]
 # This feature is kept for backwards compatibility. Use feature "serde" instead.
 eders = [ "serde" ]
 
 [dependencies]
+allocator-api2 = "0.2.20"
 serde = { version = "1.0", features = [ "derive" ], optional = true }