convert more functions to constexpr

[anonrig]

Before

BasicBench_AdaURL_url                201873 ns       201789 ns         3606 GHz=4.79146 cycle/byte=44.5044 cycles/url=1.07521k instructions/byte=111.317 instructions/cycle=2.50126 instructions/ns=11.9847 instructions/url=2.68938k ns/url=224.401 speed=102.008M/s time/byte=9.80319ns time/url=236.842ns url/s=4.22223M/s

After

BasicBench_AdaURL_url                195539 ns       195521 ns         3472 GHz=4.79119 cycle/byte=44.3816 cycles/url=1.07224k instructions/byte=111.317 instructions/cycle=2.50818 instructions/ns=12.0171 instructions/url=2.68937k ns/url=223.795 speed=105.278M/s time/byte=9.49867ns time/url=229.484ns url/s=4.3576M/s

[lemire]

Let me see why it fails.

[lemire]

See #738