Introduce AbstractPos

This makes the position object used in exceptions abstract, with a
method getSource() to get the source code of the file in which the
error originated. This is needed for lazy trees because source files
don't necessarily exist in the filesystem, and we don't want to make
libutil depend on the InputAccessor type in libfetcher.

tests/function-trace.sh

@@ -32,40 +32,40 @@ expect_trace() {
 
 # failure inside a tryEval
 expect_trace 'builtins.tryEval (throw "example")' "
-function-trace entered (string):1:1 at
-function-trace entered (string):1:19 at
-function-trace exited (string):1:19 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace entered «string»:1:19 at
+function-trace exited «string»:1:19 at
+function-trace exited «string»:1:1 at
 "
 
 # Missing argument to a formal function
 expect_trace '({ x }: x) { }' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Too many arguments to a formal function
 expect_trace '({ x }: x) { x = "x"; y = "y"; }' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Not enough arguments to a lambda
 expect_trace '(x: y: x + y) 1' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Too many arguments to a lambda
 expect_trace '(x: x) 1 2' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Not a function
 expect_trace '1 2' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 set -e