Fixes #2901.

Criteria:

• the source element type is integral
• the target element type is an EcharT (encoded character type, i.e. char, wchar_t, char8_t, char16_t, char32_t)
• the source and target element types are the same size
• the target traits type is library-provided (thanks @CaseyCarter)
• the source iterator is a contiguous iterator which doesn't dereference to a volatile lvalue.

Perhaps we can extend the criteria to move_iterator and unscoped enumeration types later.

Clang's __builtin_memcpy doesn't work when copying a char array to a char8_t array in constant evaluation (nor does GCC's), so this PR isn't using it in the involved construction.

The test P1206R7_string_from_range is largely extended to cover some related contiguous ranges and character types. The original test_c_array seemed to be a duplicate of test_copyable_views, so I changed it to properly cover built-in arrays.