linear: s/constexpr/AM_CONSTEXPR/.
hash: added constexpr implementations; tidy.

hash: s/constexpr/AM_CONSTEXPR/; tidied internals.

hash/fnv:
    added constexpr implementations;
    added fnv1a literals;
    doc tidy.

hash/murmur: added MurmurHash3 constexpr implementation.

test/hash: added constexpr and literal tests.
doc/pages: 80-column wrapping.