Changing ByCodepoint's "save" signature

1) Because if it returns a ref, then it isn't actually saving anything...
2) Changed the "const" qualifier to inout: Using "const" means the return type is always "const ByCodepoint", which isn't a valid range (a const range can't be popped)

The change is mostly moot, since it would appear ByCodepoint's save or forwardness is never used anyways. But that doesn't mean it shouldn't be correct.
imporvements to uninitializedFill && initializeAll

Because they didn't really handle all of the cases correctly.

Added unittests.
Refactored endsWith to use retro + startsWith.

What's the point of having all these powerful, generic, abstract functions if we're just going to copy and paste code? :-)

This change has several benefits:

* It is far more elegant.
* Changes (optimisations) to startsWith automatically carry across to endsWith.
* We get extra testing for startsWith and retro for free.
Gives sequence slice to end
Fixed typos in changelog.
Added not to changelog about format working in CTFE now.
Fix issue 8061.

We cannot assume that RoR.save will also .save the state of its
subranges; so the only way we can correctly export a .save method in the
joined range is if we also .save its subranges as we traverse over them.
No need to use .save on inner range if we aren't going to export .save.
FormatError => FormatException

Because it's deprecated.
representation unittest
missing import
Merge pull request #1023 from monarchdodra/extend

Minor tweaks in std.string
performance tweaks for countUntil
Merge pull request #1019 from quickfur/joiner_save

Fix issue 8061.
Merge pull request #1013 from Poita/endsWithRefactor

Refactored endsWith to use retro + startsWith.
Merge pull request #1006 from monarchdodra/internalunisave

Changing ByCodepoint's "save" signature
Merge pull request #991 from monarchdodra/dollartoken

slice to end DollarToken object, and slicing to end with Sequence
Additional fix for issue 6905

For the inference of return value ref-ness, we should use 'auto ref' instead of 'ref' or 'const ref'
Merge pull request #1028 from 9rnsr/fix6905

Additional fix for issue 6905
Merge pull request #1024 from monarchdodra/countStarts

performance tweaks for countUntil