Make time_now optional

[hhugo]

This PR is trying to reduce the dependency cone of ppx_expect/ppx_inline_tests. It turns out that the dependency on time_now is responsible (transitively) for many additional deps[1].

I propose to make time_now an optional dependency and fallback to Sys.time if time_now is not available/installed.

Note that time_now is used to log duration in seconds in -verbose mode.

[1]

• ppx_cold
• ppx_compare
• ppx_enumerate
• ppx_globalize
• ppx_hash
• ppx_sexp_conv
• jst-config
• jane-street-headers
• ppx_base
• ppx_optcomp
• ppxlib_jane