Strings & I/O (C++20)

Adds starts_with and ends_with to string and string_view.

Adds the emit_on_flush/noemit_on_flush/flush_emit manipulators.

Makes string::reserve never shrink.

Specifies the fpos requirements.

Fixes operator>>(basic_istream&, CharT*) to take a sized array, preventing overflow.

Cleans up the strings clause.

Makes stateful allocator propagation consistent for operator+ on strings.

Fixes istream_iterator construction and end-of-stream semantics.

Adds efficient access to basic_stringbuf's buffer (view(), move-out str()).

Gives string_view a range constructor.