Convert between BigInts and BigUints
Forgot to make accept() home for IO
Make I/O tests use run_in_mt_newsched_task to get more multi-threaded test coverage
Add with_mem_writer to std::rt::io::mem.

This is in many ways a replacement for the current
std::io::with_str_writer.
Fix pretty-printing of empty impl items
Convert between BigInts, BigUints, ints, and uints

Previously, conversion to ints, uints, and BigUints clamped the value
within the range of that datatype. With this commit, conversion
overflows fail the task. To handle overflows gracefully, use the new
to_*_opt() methods.
auto merge of #8999 : anasazi/rust/multi-threaded-io-tests, r=brson

Resolves #8685
Properly encode/decode structural variants.
Rename encode_struct_field_names to encode_struct_fields to reflect what it actually does
Add a test for cross-crate struct variants
Replace dashes in the filenames of the new tests with underscores to avoid issues with Windows
Fix whitespace in tests
xfail-fast the new test for windows compatibility
auto merge of #9038 : singingboyo/rust/with-mem-writer, r=anasazi

This is in many ways a replacement for the current std::io::with_str_writer.
auto merge of #9064 : SiegeLord/rust/external_struct_variants, r=luqmana

Fixes issues #5557 and #8746.

This patch adds an additional family for struct-like variants, and encodes some struct-like aspects of such variants that can then be properly decoded by resolve.

Note that I am not 100% sure how this fix works, but it fixes the issue without breaking any of the tests on my machine.
auto merge of #9014 : dcrewi/rust/convert-between-bigints, r=anasazi
Fix the empty-impl tests

Use an existing type so that it compiles.
auto merge of #9061 : jakub-/rust/pretty-print-empty-impl, r=huonw
std: rename Option::unwrap_or_default() to unwrap_or()
std: add default implementations to HashMap