chore: Improve Rust code.

[huacnlee]

• Use string to push char to reduce memory alloc.
• Use LazyLock to make static font loader for reduce alloc Font.
• Inline some short functions.

Benchmarks

Before

Warming up --------------------------------------
      Generate image    83.000 i/100ms
Calculating -------------------------------------
      Generate image    834.449 (± 1.8%) i/s    (1.20 ms/i) -      4.233k in   5.074422s

After

Warming up --------------------------------------
      Generate image    84.000 i/100ms
Calculating -------------------------------------
      Generate image    859.075 (± 1.5%) i/s    (1.16 ms/i) -      4.368k in   5.085775s