feat[lang]: change the signature of block.prevrandao

`block.prevrandao` is an opcode alias for `difficulty`. however, it
returns a bytes object at the evm level (cf. for instance py-evm returns
the value of `block.mixhash`). this commit changes the signature of
block.prevrandao to be more consistent with the VM semantics.

tests/functional/syntax/test_block.py

@@ -138,7 +138,7 @@ def foo():
     """
 @external
 def foo():
-    x: uint256 = block.prevrandao + 185
+    x: bytes32 = block.prevrandao
     if tx.origin == self:
         y: Bytes[35] = concat(block.prevhash, b"dog")
     """,

vyper/codegen/expr.py

@@ -273,7 +273,7 @@ def parse_Attribute(self):
                     warning = "tried to use block.prevrandao in pre-Paris "
                     warning += "environment! Suggest using block.difficulty instead."
                     vyper_warn(warning, self.expr)
-                return IRnode.from_list(["prevrandao"], typ=UINT256_T)
+                return IRnode.from_list(["prevrandao"], typ=BYTES32_T)
             elif key == "block.difficulty":
                 if version_check(begin="paris"):
                     warning = "tried to use block.difficulty in post-Paris "

vyper/semantics/environment.py

@@ -19,7 +19,7 @@ class _Block(_EnvType):
     _type_members = {
         "coinbase": AddressT(),
         "difficulty": UINT256_T,
-        "prevrandao": UINT256_T,
+        "prevrandao": BYTES32_T,
         "number": UINT256_T,
         "gaslimit": UINT256_T,
         "basefee": UINT256_T,