Document traits in std::num (#7511)
librustc: Improve error message for missing feature attributes.
libstd: Add unwrap_or and unwrap_or_handle to Result
libsyntax: update helper to stringify TyU* and TyI* to take into account having a value.

Fixes #13359.
auto merge of #13471 : Ryman/rust/feature_syntax_error, r=brson

The current error message is misleading, it asks users to add `#[feature(..)]` which ends up being treated as an outer attribute, which then has no error unless `attribute_usage` lint is enforced. The code will still fail and the user might not understand why.
auto merge of #13452 : Ryman/rust/fix_uint_as_u, r=alexcrichton

Fixes #13359.
rustdoc: Fix rendering closures and trait bounds

Closures did not have their bounds printed at all, nor their lifetimes. Trait
bounds were also printed in angle brackets rather than after a colon with a '+'
inbetween them.

Note that on the current task::spawn [1] documentation page, there is no mention
of a `Send` bound even though it is crucially important!

[1] - http://static.rust-lang.org/doc/master/std/task/fn.task.html
auto merge of #13470 : Manishearth/rust/docnum, r=brson

See #7511
auto merge of #13464 : alexcrichton/rust/fix-rustdoc-rendering, r=brson

Closures did not have their bounds printed at all, nor their lifetimes. Trait
bounds were also printed in angle brackets rather than after a colon with a '+'
inbetween them.

Note that on the current task::spawn [1] documentation page, there is no mention
of a `Send` bound even though it is crucially important!

[1] - http://static.rust-lang.org/doc/master/std/task/fn.task.html
auto merge of #13475 : Ryman/rust/result_unwrap_or_else, r=brson

It might make more sense to mirror `Option`'s `unwrap_or_else` but I've left it as `handle` as it feels more explicit about the signature difference.