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.

build_odin.sh

@@ -102,7 +102,7 @@ build_odin() {
 }
 
 run_demo() {
-	./odin run examples/demo/demo.odin -file
+	./odin run examples/demo
 }
 
 case $OS in
@@ -147,4 +147,4 @@ if [[ $# -eq 1 ]]; then
 	exit 0
 else
 	panic "Too many arguments!"
-fi
+fi