Only build LLVM tools Rust needs.
Update docs index to use lists

Also include a missing link to the rustdoc manual
Fixing a typo: bookeeping -> bookkeeping
Fix some docs in std::rt::task
Stop returning error strings in From{Base64,Hex}

An enum allows callers to deal with errors in a more reasonable way.
auto merge of #11588 : am0d/rust/dox, r=brson

This is a bit cleaner and means we don't need whitespace at the end of each line to force line breaks.
auto merge of #11590 : vadimcn/rust/llvm-tools, r=alexcrichton

Closes #10893 - because we'll no longer build llvm-c-test
Correct span for ExprCall and ExprIndex
auto merge of #11596 : derekchiang/rust/fix-libnative-docs, r=alexcrichton
auto merge of #11597 : sfackler/rust/err-enums, r=alexcrichton

An enum allows callers to deal with errors in a more reasonable way.
auto merge of #11599 : sanxiyn/rust/accurate-span-3, r=luqmana
auto merge of #11571 : derekchiang/rust/fix-task-docs, r=alexcrichton

There might be a reason, but I failed to see why these comments couldn't be proper rust docs.