Use heap indirection to manage type signatures

The proxy's release debug symbols are currently >6GB. It doesn't appear
to even be possible to build the proxy with debuginfo=2, as we overflow
u32::MAX and violate the restrictions of DWARF32.

To fix this, we more aggressively use dynamic dispatch via the new
`Stack::arc_new_box` helpers. This helps reduce our largest type
signatures from >700KB to ~50KB. We'll want to follow up with changes
that monitor the size of these signatures to prevent regressions.

This allows us to minimize many type assertions, etc.

There are no functional changes in this PR. Release builds will now
produce full debug information with file:line annotations.

Cargo.toml

@@ -71,5 +71,5 @@ members = [
 ]
 
 [profile.release]
-debug = 1
+debug = 2
 lto = true

Dockerfile

@@ -29,7 +29,7 @@ COPY . .
 RUN --mount=type=cache,id=cargo,target=/usr/local/cargo/registry \
     just fetch
 ENV CARGO_INCREMENTAL=0
-ENV RUSTFLAGS="-D warnings -A deprecated -C debuginfo=2"
+ENV RUSTFLAGS="-D warnings -A deprecated"
 ARG TARGETARCH="amd64"
 ARG PROFILE="release"
 ARG LINKERD2_PROXY_VERSION=""