This change rewrites most of the code in ngtcp2_ksl and removes the
need for inf_key.  It also removes the restriction that the key must
exist in descendants.  ngtcp2_ksl now embeds arbitrary sized key in
ngtcp2_node.

ngtcp2_ksl is extended so that it can work with ngtcp2_range key.
ngtcp2_psl is removed in favor of ngtcp2_ksl.

Based on nghttp2 cmake build system. Advantage of this is a faster build
(cmake runs faster than configure and ninja is faster than make).

Changes from autotools:
* Warnings are enabled by default (independent of ENABLE_WERROR).
* ENABLE_WERROR has changed meaning (it just controls -Werror).
* include/ngtcp2/{version,ngtcp2}.h header files are installed.