When the source pointer aliases the string's own buffer,
char_traits::copy (memcpy) is called on overlapping regions,
which is undefined behaviour.  Detect aliasing with
ptr_in_range and use char_traits::move (memmove) followed by
term() instead of delegating to impl_.assign, which would
write a null terminator that corrupts the source before the
copy.

The handler concept documents on_bool as taking a string_view of the
remaining characters, but its declaration is
`bool on_bool( bool b, error_code& ec )`. The line is a copy of the one
in on_double above it, which really does take one. on_null below has no
such line, so this is the only handler affected.