Symbol table now keeps track of label statements

Before it kept track of a more specific node that referenced the symbol
in some way. Now it will only keep track of the actual label defining
statements. This is done to facilitate encoding. The encoder can now go
from a symbol name to the statement that defines the symbol.

Restructure the encoder to deal with this and pass the correct statement
to the symbol update function.

src/main.c

@@ -74,11 +74,11 @@ error_t *print_encoding(tokenlist_t *list) {
         return result.err;
 
     encoder_t *encoder;
-    error_t *err = encoder_alloc(&encoder);
+    error_t *err = encoder_alloc(&encoder, result.node);
     if (err)
         goto cleanup_ast;
 
-    err = encoder_encode(encoder, result.node);
+    err = encoder_encode(encoder);
     if (err)
         goto cleanup_ast;