Zeroizing allocator (#154)

* Add a zeroizing allocator

* Also implement the allocator API

* Add misisng license file to zalloc

* Slight change to zalloc description

common/zalloc/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "zalloc"
+version = "0.1.0"
+description = "An allocator wrapper which zeroizes memory on dealloc"
+license = "MIT"
+repository = "https://github.com/serai-dex/serai/tree/develop/common/zalloc"
+authors = ["Luke Parker <lukeparker5132@gmail.com>"]
+keywords = []
+edition = "2021"
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]
+
+[dependencies]
+zeroize = "1.5"
+
+[features]
+allocator = []