Don't let unstable-locales imply the alloc feature

[pitdicker]

I hope that at some point we add support for parsing different locales (should not be all that hard). And that we can make the new formatting API work without alloc (proven to be possible in #1163).

In both cases it does not make much sense for the unstable-locales feature to imply the alloc feature.

This will break uses of chrono with no-default-features with the unstable-locales feature, that happen to also depend on alloc.
I feel we are allowed to make this change because of the name unstable-locales.

Split out from #1163.

[codecov]

Codecov Report

Merging #1307 (94d9e65) into 0.4.x (41e34ad) will not change coverage.
Report is 1 commits behind head on 0.4.x.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##            0.4.x    #1307   +/-   ##
=======================================
  Coverage   91.00%   91.00%           
=======================================
  Files          38       38           
  Lines       17068    17068           
=======================================
  Hits        15533    15533           
  Misses       1535     1535           

Files Changed	Coverage Δ
src/date.rs	35.86% <ø> (ø)
src/datetime/tests.rs	96.50% <ø> (ø)
src/format/formatting.rs	87.83% <ø> (ø)
src/format/mod.rs	85.04% <ø> (ø)
src/format/strftime.rs	98.94% <ø> (ø)
src/naive/datetime/tests.rs	98.63% <ø> (ø)
src/naive/isoweek.rs	98.16% <ø> (ø)
src/datetime/mod.rs	86.25% <100.00%> (ø)
src/format/parse.rs	97.24% <100.00%> (ø)
src/lib.rs	95.58% <100.00%> (ø)
... and 3 more

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[pitdicker]

All localized formatting methods now also depend on alloc or std.
StrftimeItems::new_with_locale becomes available on no_std.

[djc]

I'm okay with this, but any(feature = "alloc", feature = "std") is silly because std should imply alloc.

[pitdicker]

I'm okay with this, but any(feature = "alloc", feature = "std") is silly because std should imply alloc.

It did not bother me enough yet 😄. I have added a commit to clean that up.

[pitdicker]

Nice, this also reduced the number of feature combinations the CI tests with 35%.