Auto merge of #27871 - alexcrichton:stabilize-libcore, r=aturon

These commits move libcore into a state so that it's ready for stabilization, performing some minor cleanup:

* The primitive modules for integers in the standard library were all removed from the source tree as they were just straight reexports of the libcore variants.
* The `core::atomic` module now lives in `core::sync::atomic`. The `core::sync` module is otherwise empty, but ripe for expansion!
* The `core::prelude::v1` module was stabilized after auditing that it is a subset of the standard library's prelude plus some primitive extension traits (char, str, and slice)
* Some unstable-hacks for float parsing errors were shifted around to not use the same unstable hacks (e.g. the `flt2dec` module is now used for "privacy").


After this commit, the remaining large unstable functionality specific to libcore is:

* `raw`, `intrinsics`, `nonzero`, `array`, `panicking`, `simd` -- these modules are all unstable or not reexported in the standard library, so they're just remaining in the same status quo as before
* `num::Float` - this extension trait for floats needs to be audited for functionality (much of that is happening in #27823)  and may also want to be renamed to `FloatExt` or `F32Ext`/`F64Ext`.
* Should the extension traits for primitives be stabilized in libcore?

I believe other unstable pieces are not isolated to just libcore but also affect the standard library.

cc #27701

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