read: add support for custom address types

Add the `Reader::Address` associated type, which implements the
`ReaderAddress` trait. This trait is implemented for u64 to
provide the existing behaviour of supporting multiple address sizes,
as well as for `Address32` and `Address64` which support a single
address size.

The dwarfdump example now provides its own address type, which
allows it to show symbols associated with relocated addresses
in relocatable object files.
For example:

    DW_AT_low_pc                main+0x0
    DW_AT_ranges                <rnglist at .debug_rnglists+0x0000000c>
        [ 0] <start-length main+0x0, 0x17> [main+0x0, main+0x17]

    DW_LNE_set_address to main+0x0

benches/bench.rs

@@ -468,7 +468,7 @@ fn bench_evaluating_debug_info_expressions(b: &mut test::Bencher) {
     });
 }
 
-fn debug_loc_expressions<R: Reader>(
+fn debug_loc_expressions<R: Reader<Address = u64>>(
     debug_info: &DebugInfo<R>,
     debug_abbrev: &DebugAbbrev<R>,
     debug_addr: &DebugAddr<R>,
@@ -702,7 +702,7 @@ mod cfi {
         });
     }
 
-    fn instrs_len<R: Reader>(
+    fn instrs_len<R: Reader<Address = u64>>(
         eh_frame: &EhFrame<R>,
         bases: &BaseAddresses,
         fde: &FrameDescriptionEntry<R>,
@@ -712,7 +712,7 @@ mod cfi {
             .expect("fold over instructions OK")
     }
 
-    fn get_fde_with_longest_cfi_instructions<R: Reader>(
+    fn get_fde_with_longest_cfi_instructions<R: Reader<Address = u64>>(
         eh_frame: &EhFrame<R>,
         bases: &BaseAddresses,
     ) -> FrameDescriptionEntry<R> {