Add chained fallback support and performance improved

[varphone]

New:

• Add more than one fallback with priority support, eg: i18n!("locales", fallback = ["en", "es]);
  • More than one fallback with priority #40

Improved:

• Remove RwLock from locale() and set_locale().
• String patterns replacement, time reduce 10% ~ 80%.
• Reduce memory copy on t!().

Breaking Changes:

• rust_i18n::locale() -> String => rust_i18n::locale() -> Arc<String> .
• t!() -> String => t!() -> Cow<str>.

[sunli829]

rust_i18n::locale() -> String => rust_i18n::locale() -> Arc<str>

Arc<str> is better than Arc<String>, what do you think?

[huacnlee]

Update this new benchmark result to README.md

t                       time:   [58.274 ns 60.222 ns 62.390 ns]
t_with_locale           time:   [55.395 ns 57.106 ns 59.081 ns]
t_with_args             time:   [167.46 ns 170.94 ns 175.64 ns]
t_with_args (str)       time:   [164.85 ns 165.91 ns 167.41 ns]
t_with_args (many)      time:   [444.04 ns 452.17 ns 463.44 ns]
t_with_threads          time:   [414.26 ns 422.97 ns 433.53 ns]

[varphone]

rust_i18n::locale() -> String => rust_i18n::locale() -> Arc<str>

Arc<str> is better than Arc<String>, what do you think?

The AtomicPtr

rust_i18n::locale() -> String => rust_i18n::locale() -> Arc<str>

Arc<str> is better than Arc<String>, what do you think?

AtomicStr needs to hold the string content, which must be stored using a String.
Putting the String into Arc can achieve zero-copy reference, so returning Arc<String> directly is the most efficient way.
If use Arc<str> as the return the value will produce a memory copy.

• https://doc.rust-lang.org/src/alloc/sync.rs.html#3315
• https://doc.rust-lang.org/src/alloc/sync.rs.html#3334

[huacnlee]

https://github.com/longbridgeapp/rust-i18n/releases/tag/v2.4.0