Make the various from_str functions return options

So that they can be used with user input without causing task
failures.

Closes #1335

src/comp/syntax/ast_util.rs

@@ -301,7 +301,7 @@ fn lit_to_const(lit: @lit) -> const_val {
       lit_str(s) { const_str(s) }
       lit_int(n, _) { const_int(n) }
       lit_uint(n, _) { const_uint(n) }
-      lit_float(n, _) { const_float(float::from_str(n)) }
+      lit_float(n, _) { const_float(option::get(float::from_str(n))) }
       lit_nil { const_int(0i64) }
       lit_bool(b) { const_int(b as i64) }
     }