…r one

Reason: See the new code comment.

…texpr context

Reason: They have the same issue as GCC 9.

They were conditioned on detection of C++20 via __cplusplus, but Clang
10 and 11 don't support class types as NTTP, even though they report
C++20 via __cplusplus when -std=c++20 is used.

The iterator-based insert(const_iterator, size_type, value_type)
function relies on traits_type::move() to shift the existing null
terminator to its new position. Clang 3.7's constexpr evaluator does not
handle this correctly, causing the following test to fail:

  static_string<3>{"ab"}.insert(2, 1, 'c') == "abc"

Add an explicit term() call, guarded by a preprocessor conditional for
Clang 3.7, to ensure proper null termination.

This introduces a lightweight alternative to basic_static_string
designed for use in POD types: trivially copyable, sizeof == N + 1,
no embedded NULs.

Placed in example/ to gather user feedback before committing to a public
API. See <#23>.