Optimization: Compare class name directly

Previously, using str() parsed and caused a large
overhead (this is 91% more efficient per my janky benchmark!)

src/driftpy/types.py

@@ -7,11 +7,12 @@
 import zlib
 
 from borsh_construct.enum import _rust_enum
+from solders.pubkey import Pubkey
 from sumtypes import constructor
 
 
 def is_variant(enum, type: str) -> bool:
-    return type in str(enum)
+    return type == enum.__class__.__name__
 
 
 def is_one_of_variant(enum, types):