Do not use optimised debug builds

Optimised debug builds result in an extremely slow code-build-test
cycle.  The reason we do this is because we have a few tests which end
up overflowing the stack.  This increases the stack instead.

.cargo/config.toml

@@ -0,0 +1,9 @@
+[env]
+# In unoptimised builds tokio tends to use a lot of stack space when
+# creating some complicated futures.  Some of our tests manage to not
+# fit in the default 2MiB stack anymore due to this.  Because
+# compiling optimised builds takes a very long time we prefer to avoid
+# that.  Setting this environment variable ensures that when invoking
+# `cargo test` threads are allowed to have a large enough stack size
+# without needing to use an optimised build.
+RUST_MIN_STACK = "8388608"

Cargo.toml

@@ -9,7 +9,7 @@ rust-version = "1.56"
 [profile.dev]
 debug = 0
 panic = 'abort'
-opt-level = 1
+opt-level = 0
 
 [profile.release]
 lto = true