optimize identity precompile (vyperlang#2488)

If we use staticcall, we don't need to pass value=0, saving 2 bytes in
bytecode / 3 gas at runtime

Also, we do not need to have an assert after call to identity (the only
way failure can happen is OOG and by that time it is too late to do
anything anyway).

Related: vyperlang#1929, vyperlang#1930

vyper/old_codegen/parser_utils.py

@@ -111,7 +111,7 @@ def make_byte_array_copier(destination, source, pos=None):
     if destination.location == "memory" and source.location in ("memory", "code", "calldata"):
         if source.location == "memory":
             # TODO turn this into an LLL macro: memorycopy
-            copy_op = ["assert", ["call", ["gas"], 4, 0, "src", "sz", destination, "sz"]]
+            copy_op = ["staticcall", "gas", 4, "src", "sz", destination, "sz"]
             gas_bound = _identity_gas_bound(source.typ.maxlen)
         elif source.location == "calldata":
             copy_op = ["calldatacopy", destination, "src", "sz"]
@@ -179,7 +179,7 @@ def make_byte_slice_copier(destination, source, length, max_length, pos=None):
                 "with",
                 "_l",
                 max_length,  # CMC 20210917 shouldn't this just be length
-                ["pop", ["call", ["gas"], 4, 0, source, "_l", destination, "_l"]],
+                ["staticcall", "gas", 4, source, "_l", destination, "_l"]
             ],
             typ=None,
             annotation=f"copy byte slice dest: {str(destination)}",