Move in-place functions below their iterator variants.
Place union as the first function, for consistency.
Group union, intersect and difference in Bitv.
Document Bitv.
Document BitvSet.
Move intersection above difference and symmetric_differance.

So all comes in the order union, intersection, difference and
symmetric_difference.
Main bitv example: prime sieve.
Enclose None as `None`.
Polish bitv docs.
Describe BitPositions and TwoBitPositions.
Simplify and cleanup bitv examples.
syntax: Join consecutive string literals in format strings together

Emit a single rt::Piece per consecutive string literals. String literals
are split on {{ or }} escapes.

Saves a small amount of static storage and emitted code size.
Update LLVM to include NullCheckElimination pass

Fixes #11751
Rename to_str to to_string

Closes #15796.

[breaking-change]
Remove useless RefCells
Restructure test harness

We now build up a set of modules that reexport everything the test
framework needs, instead of turning off privacy.
Purge !resolve_unexported
Don't create reexport module if there are none
privacy: Add publically-reexported foreign item to exported item set

Close #15740
fix string in from_utf8_lossy_100_multibyte benchmark