Make once_cell dependency optional

[tatsuya6502]

Fixes #427.

This pull request changes once_cell crate optional. After applying this change, once_cell is used by moka only when the followings:

• development build (e.g. cargo test).
• unstable-debug-counters feature is enabled.

## `once_cell` is used in [dev-dependency], and indirectly via `quanta`.

$ cargo tree -i once_cell -F future,sync
once_cell v1.19.0
├── ahash v0.8.11
│   [dev-dependencies]
│   └── moka v0.12.8 ( ... /moka)
├── async-global-executor v2.4.1
│   └── async-std v1.12.0
│       [dev-dependencies]
│       └── moka v0.12.8 ( ... /moka)
├── async-std v1.12.0 (*)
├── quanta v0.12.3
│   └── moka v0.12.8 ( ... /moka)
├── reqwest v0.11.27
│   [dev-dependencies]
│   └── moka v0.12.8 ( ... /moka)
└── tracing-core v0.1.32
    └── tracing v0.1.40
        ├── async-io v2.3.3
        │   └── async-global-executor v2.4.1 (*)
        ├── h2 v0.3.26
        │   ├── hyper v0.14.30
        │   │   ├── hyper-rustls v0.24.2
        │   │   │   └── reqwest v0.11.27 (*)
        │   │   └── reqwest v0.11.27 (*)
        │   └── reqwest v0.11.27 (*)
        ├── hyper v0.14.30 (*)
        └── polling v3.7.2
            └── async-io v2.3.3 (*)
[dev-dependencies]
└── moka v0.12.8 ( ... /moka)

## If the default features is disabled, `once_cell` appears only under [dev-dependency].

cargo tree -i once_cell --no-default-features -F future,sync
once_cell v1.19.0
├── ahash v0.8.11
│   [dev-dependencies]
│   └── moka v0.12.8 ( ... /moka)
├── async-global-executor v2.4.1
│   └── async-std v1.12.0
│       [dev-dependencies]
│       └── moka v0.12.8 ( ... /moka)
├── async-std v1.12.0 (*)
├── reqwest v0.11.27
│   [dev-dependencies]
│   └── moka v0.12.8 ( ... /moka)
└── tracing-core v0.1.32
    └── tracing v0.1.40
        ├── async-io v2.3.3
        │   └── async-global-executor v2.4.1 (*)
        ├── h2 v0.3.26
        │   ├── hyper v0.14.30
        │   │   ├── hyper-rustls v0.24.2
        │   │   │   └── reqwest v0.11.27 (*)
        │   │   └── reqwest v0.11.27 (*)
        │   └── reqwest v0.11.27 (*)
        ├── hyper v0.14.30 (*)
        └── polling v3.7.2
            └── async-io v2.3.3 (*)
[dev-dependencies]
└── moka v0.12.8 ( ... /moka)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.04%. Comparing base (1636ed9) to head (19abacf).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
+ Coverage   95.00%   95.04%   +0.03%     
==========================================
  Files          44       44              
  Lines       20979    20979              
==========================================
+ Hits        19932    19939       +7     
+ Misses       1047     1040       -7     

[tatsuya6502]

Merging.