encode trait lifetime params in metadata to allow cross-crate usage

Before applying this patch, the included testcase fails with:
```
src/test/run-pass/xcrate-trait-lifetime-param.rs:20:10: 20:28 error: wrong number of lifetime parameters: expected 0 but found 1
src/test/run-pass/xcrate-trait-lifetime-param.rs:20 impl <'a> other::FromBuf<'a> for Reader<'a> {
                                                              ^~~~~~~~~~~~~~~~~~
```

There's another example in my comments to #10506.