Replace std::string_view of c++17 with c++11 compatible string-view-lite

[clover2123]

Performance result: there is no impact of this patch

test	walrus_patch [s]	origin_walrus [s]
change	2.598 (1.000x)	2.583 (0.994x)
factorial	2.708 (1.000x)	2.752 (1.016x)
fannkuch	2.254 (1.000x)	2.314 (1.026x)
fibonacci	3.805 (1.000x)	3.793 (0.997x)
gregory	3.014 (1.000x)	3.083 (1.023x)
hanoi	3.400 (1.000x)	3.382 (0.995x)
heapsort	2.492 (1.000x)	2.565 (1.029x)
huffman	3.520 (1.000x)	3.530 (1.003x)
k_nucleotide	2.934 (1.000x)	3.006 (1.025x)
mandelbrot	3.866 (1.000x)	3.967 (1.026x)
nbody	3.290 (1.000x)	3.294 (1.001x)
nqueens	3.101 (1.000x)	3.111 (1.003x)
prime	3.338 (1.000x)	3.368 (1.009x)
quick_sort	2.692 (1.000x)	2.659 (0.988x)
red-black	3.614 (1.000x)	3.685 (1.020x)
salesman	3.125 (1.000x)	3.117 (0.997x)

[clover2123]

Get c++11 string-view-list from https://github.com/martinmoene/string-view-lite with Boost Software License 1.0

[clover2123]

License check done and add BSL-1.0 license file