Summary

This PR optimizes the performance of the append function in include/fmt/base.h.
This performance improvement was identified while profiling the spdlog logging library, a major downstream project that bundles fmt. The benchmarks show that this optimization provides significant gains in spdlog's throughput and latency. For context, this change was first submitted to spdlog (see gabime/spdlog#3465) where I was advised to contribute it upstream directly to fmt.
Across 47 test cases from spdlog's benchmark suite, this change achieves a maximum improvement of 16.67% while guaranteeing no regression exceeds 0.99% in any other case.

Test Plan

1. Correctness: All existing unit tests in the fmt project pass.
2. Performance: The performance impact was evaluated using the comprehensive benchmark suite from the spdlog project. This was chosen because spdlog is a key real-world use case, and its benchmarks effectively measure the performance of fmt's core formatting operations under various conditions (single-threaded, multi-threaded, different logging patterns). The commands used were ./bench/bench and ./bench/latency from the spdlog repository.

Performance Evaluation & Results

Testing Protocol:

• The benchmark was run on an isolated Ubuntu 24.04 server using the spdlog benchmark suite.
• The first run was discarded to account for cold-start effects.
• The results below are the average of 5 subsequent runs.
• The improvement for a test case is calculated as (new_value - old_value) / old_value * 100% if a higher value is better (e.g., throughput), or (old_value - new_value) / new_value * 100% if a lower value is better (e.g., latency). A positive percentage indicates a performance gain.

Results: