Switch Rust global allocator to tcmalloc.

[dmkozh]

Description

Switch Rust global allocator to tcmalloc.

tcmalloc yielded performance improvements for C++, and it's also significantly better than the system allocator on the apply time benchmarks (~30-50ms of apply time savings on large TPL benchmarks, like 6000 SAC TPS).

Rust implementation relies on the tcmalloc functions already linked into the Core binary. That's why we're using a custom allocator implementation instead of using an existing tcmalloc wrapper for Rust.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[Copilot]

Pull request overview

This PR switches the Rust crate’s global allocator to use the C++-linked tcmalloc (gperftools) allocator when the build is configured with USE_TCMALLOC, aiming to improve performance by routing Rust (and embedded Soroban host) allocations through tcmalloc.

Changes:

• Added a Rust GlobalAlloc implementation (TcMalloc) that forwards allocations to tc_* symbols provided by libtcmalloc_minimal.a.
• Enabled the allocator via a new Cargo feature (tcmalloc) and a #[global_allocator] in the top-level Rust crate.
• Updated the autotools build (src/Makefile.am) to pass --features tcmalloc to the Rust build whenever USE_TCMALLOC is set.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/rust/src/tcmalloc.rs	Introduces a tcmalloc-backed Rust GlobalAlloc implementation.
src/rust/src/lib.rs	Enables TcMalloc as the Rust global allocator under the tcmalloc feature.
src/rust/Cargo.toml	Adds the tcmalloc feature flag for the Rust crate.
src/Makefile.am	Wires USE_TCMALLOC to --features tcmalloc during Rust build invocations.

[graydon]

LGTM