recover_number() used back(), pop_back(), front(), and
find_first_of(const char*), which the minimal alt_string of
unit-alt-string.cpp does not provide. Since the binary readers recover
UBJSON/BJData high-precision numbers with it, from_ubjson() instantiated
it, too, and the test no longer compiled. It now uses only size(),
operator[], and resize(), and unit-alt-string.cpp recovers from errors
in JSON text, UBJSON, and CBOR.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>