Add wasm_of_ocaml benchmarks with current-bench output

[OlivierNicole]

@vouillon and I (so far, mostly @vouillon) have started to look at ways of improving performance of Wasm code. To keep track of our progress, we need to set up a benchmarking suite that gets run regularly.

This PR makes the repository compatible for monitoring by https://github.com/ocurrent/current-bench/: the benchmark suite is extended with the current-bench output format and I modified the scripts so that Wasm benchmarks are first-class citizens. The existing benchmarking utilities should still work, I ran a few checks to verify that.

This is a first step, we will soon want to add macrobenchmarks as well — presumably by having a benchmarks/sources/macro directory where each subdirectory is a macrobenchmark with its own dune file.

[OlivierNicole]

Note: this is ready for review but I still need to verify details—namely, how current-bench deals with multiple opam files, and what should be the name of the results file—so we should hold off from merging yet.