Add signature literals (#505)

* Add signature literals + handle system alias types

* Add tests for signature literals + encode/decode

* Add to CHANGELOG

* Add in documentation

* Additional documentation

test/contracts/ceres.aes

@@ -11,4 +11,5 @@ contract C =
     let i = Int.mulmod(a, b, h)
     let j = Crypto.poseidon(i, a)
     let k : bytes(32) = Address.to_bytes(Call.origin)
-    (a bor b band c bxor a << bnot b >> a, k)
+    let l = sg_MhibzTP1wWzGCTjtPFr1TiPqRJrrJqw7auvEuF5i3FdoALWqXLBDY6xxRRNUSPHK3EQTnTzF12EyspkxrSMxVHKsZeSMj
+    (a bor b band c bxor a << bnot b >> a, k, l)

test/contracts/funargs.aes

@@ -59,3 +59,5 @@ contract FunctionArguments =
   entrypoint chain_ga_meta_tx(tx : Chain.ga_meta_tx) = true
   entrypoint chain_paying_for_tx(tx : Chain.paying_for_tx) = true
   entrypoint chain_base_tx(tx : Chain.base_tx) = true
+
+  entrypoint sig(sg : signature) = true