boot: errors as values (-> T!, error(), try, or, fallible builtins and main)

A function that can fail returns { ok, value, message }. return error(...) makes
the failure; try passes it on after running the defers; 'or <value>' and
'or { ... err ... }' handle it. int(text), float(text), read_file, write_file
and delete_file report failure to an enclosing or/try instead of stopping the
program. fn main() -> ! prints the error and exits with 1.

boot.sh: 15 passed, 0 failed, 7 skipped.

boot/ast.fch

@@ -78,7 +78,8 @@ struct Node {
     str recv      // fn Point.move(...): "Point"; empty for a plain function
     str text      // literal text
     Type ty       // declared type (decl, param, field, fn result)
-    bool flag     // decl: has a type; fn: has a body
+    bool flag     // decl: has a type; or: the right side is a { block }
+    bool fallible // fn: -> T!  or  -> !
     []Node kids
     []str names   // named arguments: Point(x: 1)
     int line