Compiler: Add early error for output path being a directory.

- Introduce new `Path` type and an array of build paths on the build context.
- Resolve input and output paths/files early (before parsing).
- Error early if inputs are missing or outputs are directories.
- Plumb new file path generation into linker stage instead of its adhoc method.

TODO:
- Remove more adhoc file path generation in parser and linker stage.
- Make intermediate object file generation use new path system.
- Round out and robustify Path helper functions.

Makefile

@@ -1,7 +1,7 @@
-all: debug demo
+all: debug
 
 demo:
-	./odin run examples/demo/demo.odin
+	./odin run examples/demo
 
 report:
 	./odin report