nothrow for Schedulder

- require nothrow for thisInfo, yield

- make Condition methods nothrow
Mark std.range.cycle for static array and their unittests as system
Extend groupBy to handle non-equivalence relations.

Update DDocs.

Add unittest for 13595.
Use flag enum instead of bool.

To prevent Boolean Blindness.
Do reflexivity check in assert mode if predicate is claimed to be equivalence relation.
Use Flag instead of single-use enum.
Merge pull request #2704 from MartinNowak/nothrowScheduler

nothrow for Schedulder
Rename IsEquivRelation to EquivRelation.
make nested foreach over TypeTuple bodies into lambdas to avoid huge functions
Merge pull request #2767 from rainers/unittest_smallfuncs

Reduce compilation time for phobos unittests by about 25%
Use version-specific navbar when building docs.
use ElementType consistently
Merge pull request #2775 from e10s/patch-3

std.range.package: use ElementType consistently
Use -prerelease for navbar ddoc instead of hardcoded version.
Merge pull request #2654 from quickfur/issue13595b

Issue 13595: Extend groupBy to handle non-equivalence relations.
add upgrade hints
Merge pull request #2777 from WalterBright/upgrade

add upgrade hints
Merge pull request #2769 from tom-tan/system-range-cycle-for-sarray

Mark std.range.cycle for static array and their unittests as system
Merge pull request #2774 from quickfur/versioned_navbar

Use version-specific navbar when building docs.