Create perf-helpers subproject to help in running code profiling

[macisamuele]

The goal of this PR is to streamline the process of getting obtaining a flamegraph to focus optimisation efforts (as described on #91 (comment))

The cargo-flamegraph crate allows to run a binary with perf/dtrace (depending on the platform) and producing a SVG file containing the flamegraph.

In order to obtain it we need to have debug symbols in the release build (and the subproject takes care of that).

In order to make the tool easier to use I've ensured that perf-helpers/src/main.rs retrieves the schema and instance JSON representation at compile time, such that testing different combinations requires to modify the command to run instead of the file to compile.

The way of using it is

./perf-helpers/run ${schema_path} ${instance_path} [${number_of_iterations}]

The output file will be available on the directory from where you've run the script