allocator: Add optional heap allocation measurement tracking. (#38727)

Add an off-by-default allocator mode that tracks all live allocations
with sizes and associated stack traces. We also track if each allocation
is visited as part of a measuring heap usage in `about:memory`, allowing
us to report on allocations that are not tracked yet. Right now the list
of untracked allocations is dumped to stdout; I have a python script
coming in a separate PR which makes it easier to perform analysis on the
massive output.

Testing: Manually tested with `./mach build -d --features
servo_allocator/allocation-tracking` and visiting about:memory.
Part of: #11559

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/allocator/Cargo.toml

@@ -11,8 +11,13 @@ rust-version.workspace = true
 path = "lib.rs"
 
 [features]
+allocation-tracking = ["backtrace", "fnv"]
 use-system-allocator = ["libc"]
 
+[dependencies]
+backtrace = { workspace = true, optional = true }
+fnv = { workspace = true, optional = true }
+
 [target.'cfg(not(any(windows, target_env = "ohos")))'.dependencies]
 libc = { workspace = true, optional = true }
 tikv-jemalloc-sys = { workspace = true }