Update in chunks

reduces requirements to ForwardRange
Fix std.BigInt bug #9548 plus other new discovered bugs.
use default MODEL unless explicitly specified
Fixed regex loop-back stream to start from the correct position
Use ddoc-ed unittest feature
fix Issue 9976 - Needlessly large instantiation depth in std.typetuple algorithms
Merge branch 'master' of https://github.com/D-Programming-Language/phobos
Fixed a bunch of off-by-one errors in the regex code
Remove unnecessary default arguments
Removed unused stream abstraction
Make std.conv unittests independent of floating point rounding
fix issue 10167
immutable should be an enum
Merge pull request #1306 from WalterBright/toenum

immutable should be an enum
Merge pull request #1304 from majiang/patch-2

fix issue 10167
Merge pull request #1273 from Diggsey/master

fix issue 9979 - regex back-looper bug
Merge pull request #1231 from Safety0ff/master

Fix std.BigInt bug #9548 plus other new discovered bugs.
fix for calling .array() on ranges in CTFE
Merge pull request #1305 from dymk/ctfe-array

fix for calling .array() on ranges in CTFE
Fixed std.bitmanip unit test so that the intended behaviour of checking both endian types is the actual behaviour