Use lalrpop process_src() helper function

This alternative to process_root() was added in lalrpop 0.21 and
searches only in the src directory for lalrpop files, rather than the
entire project directory.

build.rs

@@ -7,7 +7,7 @@ mod casc;
 
 fn main() -> std::io::Result<()> {
     // Generate parser
-    lalrpop::process_root().unwrap();
+    lalrpop::process_src().unwrap();
 
     // Generate man page
     // https://rust-cli.github.io/book/in-depth/docs.html