fix: _cycles_per_sec might crash due to overflow by reordering of rdtsc

[BewareMyPower]

fixes #5

[tisonkun]

Generally, in a good direction. Comments inline.

[tisonkun]

nit: move this import to the block use it below. And I remember we prefer use std:: since this crate requires std.

[BewareMyPower]

Addressed. The original code is generated by LLM with full qualified path, so I told LLM to import the function or method instead.

• Use use imports instead of fully qualified paths. For example, import std::sync::atomic::compiler_fence and use compiler_fence(...) rather than std::sync::atomic::compiler_fence(...).

Now I added another convention:

• Scope use imports inside cfg blocks rather than at module top level when they are only needed there

[tisonkun]

Pushed a new commit for tidy code. LGTM.

cc @andylokandy @zhongzc

[tisonkun]

@BewareMyPower Do you have a bench report after introducing this fence like #11 (comment)?

[BewareMyPower]

introducing this fence

I assume you mentioned this PR.

The current workflow already run the bench, see https://github.com/fast/fastant/actions/runs/25777647841/job/75713505393

Here is a Python script that parses the raw outputs:
bench_table.py

Implementation	Time (ns)	vs std
fastant	9.53 ns (9.52-9.55)	0.56x faster
quanta	11.84 ns (11.83-11.86)	0.69x faster
std	17.15 ns (17.14-17.16)	baseline

The change in this PR does not matter because it only affects the warm-up phase. The fence introduced in monotonic_with_tsc, which is only called by _cycles_per_sec():

#[small_ctor::ctor] init()              // tsc_now.rs:24  (runs before main())
  → TSCLevel::get()                     // tsc_now.rs:68
    → is_tsc_stable()                   // tsc_now.rs:95  (CPUID + /sys checks)
    → cycles_per_sec(anchor)            // tsc_now.rs:150
      → _cycles_per_sec()               // tsc_now.rs:160  ← CRASH HERE

_cycles_per_sec() runs only during program initialization, before main(), because init() is annotated with #[small_ctor::ctor] which places it in the .init_array linker section.

On the hot path, Instant::now() does not call anything heavy:

Instant::now()                          // instant.rs:32
  → crate::current_cycle()             // lib.rs:61
    → tsc_now::current_cycle()         // tsc_now.rs:50
      → tsc().wrapping_sub(cycles_from_anchor)  // just reads cached values

tsc() still does not use fence, that's the key point.