Consider making `valarray` representation two pointers instead of pointer and size

[AlexGuteniev]

Advantages:

• Should not suffer from the vectorization issue that needs member caching to a local variable commented in Avoid calling size() in valarray loops #3968
• Matches libc++ representation

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.

[CaseyCarter]

• Matches libc++ representation

How is this an advantage?

[AlexGuteniev]

How is this an advantage?

Same performance expectation for different implementations means you might be able to micro-optimize while still using the STL.