Remove <no-bounds> on trait objects

Printing <no-bounds> on trait objects comes from a time when trait
objects had a non-empty default bounds set. As they no longer have any
default bounds, printing <no-bounds> is just noise.
shootout-meteor improvement

- 5-10% of raw speedup
- parallelization of the search
serialize: Ignore two flaky json tests on windows

See #14064 for some rationale, but the basic idea is that I suspect that there
is an LLVM codegen bug somewhere, and I'm not entirely sure why it's happening
intermittently rather than deterministically...

cc #14064
auto merge of #14057 : kballard/rust/remove_no-bounds, r=sfackler

Printing <no-bounds> on trait objects comes from a time when trait
objects had a non-empty default bounds set. As they no longer have any
default bounds, printing <no-bounds> is just noise.
auto merge of #14063 : TeXitoi/rust/shootout-meteor-improvement, r=pcwalton

- 5-10% of raw speedup
- parallelization of the search
auto merge of #14065 : alexcrichton/rust/ignore-flaky-windows-test, r=brson

See #14064 for some rationale, but the basic idea is that I suspect that there
is an LLVM codegen bug somewhere, and I'm not entirely sure why it's happening
intermittently rather than deterministically...

cc #14064