Add function expr parsing

* Introduce new `fn` keyword
* Function example is added to examples/expr.not

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/syn/lexer.l

@@ -1,4 +1,6 @@
 %%
+fn                          "fn"
+
 [\r\n;]+                    "EOL"
 [a-zA-Z_][a-zA-Z0-9_]*      "IDENT"
 :[a-zA-Z_][a-zA-Z0-9_]*     "SYM"
@@ -22,6 +24,8 @@
 \)      ")"
 \[      "["
 \]      "]"
+\{      "{"
+\}      "}"
 \.      "."
 ,       ","