Remove alloc keyword for struct

src/compiler/NubLang/Tokenization/Tokenizer.cs

@@ -14,7 +14,6 @@ public sealed class Tokenizer
         ["break"] = Symbol.Break,
         ["continue"] = Symbol.Continue,
         ["return"] = Symbol.Return,
-        ["alloc"] = Symbol.Alloc,
         ["struct"] = Symbol.Struct,
         ["let"] = Symbol.Let,
         ["calls"] = Symbol.Calls,