feat: add native_lisp_function macro

- refactor project layout to use child crates
  - lispers-core: parser and evaluator
  - lispers-macro: proc macros

lispers-core/src/parser/token.rs

@@ -0,0 +1,14 @@
+#[derive(Debug, PartialEq, Clone)]
+/// Sum type of different tokens
+pub enum Token {
+    FloatLiteral(f64),
+    IntLiteral(i64),
+    Dot,
+    Nil,
+    ParClose,
+    ParOpen,
+    Quote,
+    StringLiteral(String),
+    Symbol(String),
+    True,
+}