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

[dwrensha]

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.

[alexcrichton]

cc @nikomatsakis

[dwrensha]

r please?

For evidence that this pull request is a correct fix to the bug, compare to decoder::get_trait_def:

pub fn get_trait_def(cdata: Cmd,
                     item_id: ast::NodeId,
                     tcx: ty::ctxt) -> ty::TraitDef
{
    let item_doc = lookup_item(item_id, cdata.data);
    let tp_defs = item_ty_param_defs(item_doc, tcx, cdata,
                                     tag_items_data_item_ty_param_bounds);
    let rp_defs = item_region_param_defs(item_doc, tcx, cdata);
...