Implement memory.grow and memory.current (#9)

* Implement.

* Clean and doc

* Collect base addresses instead of leaking them

* Fix code for 1.25.

* Simplify $assert

* Use AbiParam::special.

* Use &mut self in base_addr

lib/execute/src/lib.rs

@@ -14,11 +14,13 @@
 
 extern crate cranelift_codegen;
 extern crate cranelift_wasm;
+extern crate memmap;
 extern crate region;
 extern crate wasmtime_environ;
 
 mod execute;
 mod instance;
+mod memory;
 
 pub use execute::{compile_and_link_module, execute};
 pub use instance::Instance;