Constructors and contract creation (#11)

Implement constructor logic and support create/create2 in the mock runtime

Signed-off-by: xermicus <cyrill@parity.io>

crates/llvm-context/src/polkavm/const/runtime_api.rs

@@ -24,6 +24,8 @@ pub static HASH_KECCAK_256: &str = "hash_keccak_256";
 
 pub static INPUT: &str = "input";
 
+pub static INSTANTIATE: &str = "instantiate";
+
 pub static NOW: &str = "now";
 
 pub static RETURN: &str = "seal_return";