const wasm_allocator: Allocator = .{ .ptr = undefined, .vtable = std.heap.WasmAllocator.vtable };

This allocator is fast, small, and specific to WebAssembly. In the future, this will be the implementation automatically selected by GeneralPurposeAllocator when compiling in ReleaseSmall mode for wasm32 and wasm64 architectures. Until then, it is available here to play with.