wasmtime-bench-api: Randomize the locations of heap objects

This helps us avoid measurement bias due to accidental locality of unrelated
heap objects. See *Stabilizer: Statistically Sound Performance Evaluation* by
Curtsinger and Berger for details (although Stabilizer deals with much more than
just the location of heap allocations):
https://people.cs.umass.edu/~emery/pubs/stabilizer-asplos13.pdf

crates/bench-api/Cargo.toml

@@ -16,10 +16,13 @@ crate-type = ["rlib", "cdylib"]
 
 [dependencies]
 anyhow = "1.0"
+shuffling-allocator = { version = "1.1.1", optional = true }
 wasmtime = { path = "../wasmtime", default-features = false }
 wasmtime-wasi = { path = "../wasi" }
 wasi-common = { path = "../wasi-common" }
 
-
 [dev-dependencies]
 wat = "1.0"
+
+[features]
+default = ["shuffling-allocator"]