Add weights benchmarks (step 1)

This is a first MR that create the structure for weights benchmarking, and officially define the reference machine as a Raspberry Pi 4 Model B - 4GB.

This MR also add computed benchmarks for pallets system, balance and universal_dividend.

docs/dev/weights-benchmarking.md

+# How to benchmarks weights of a Call/Hook/Pallet
+
+1. Create the benchmarking tests, see commit 31057e37be471e3f27d18c63818d57cc907b4b4f for a
+complete real example.
+2. Run the benchmark test on your local machine:
+`cargo test -p <pallet> --features runtime-benchmarks`
+3. If the benchmark tests compiles and pass, compile the binary with benchmarks on your local
+machine: `cargo build --release --features runtime-benchmarks`
+4. Run the benchmarks on your local machine (to test if it work mith a real runtime). The command
+is: `duniter benchmark --chain=CURRENCY-dev --steps=50 --repeat=20 --pallet=pallet_universal_dividend --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --header=./file_header.txt --output=.`
+5. If it work, use the generated file content to create or update the `WeightInfo` trait and the `()` dummy implementation. Then use the `WeightInfo` tarit in the real code of the pallet. See 79e0fd4bf3b0579279fc957da5e2fdfc6d8a17fa for a
+complete real example.
+6. Redo steps `3.` and `4.` on the reference machine.
+7. Put the generated file on `runtime/common/src/weights` and use it in the runtimes configuration.
+See cee7c3b2763ba402e807f126534d9cd39a8bd025 for a complete real example.
+
+Note 1: You Must replace `CURRENCY` by the currency type, or for ĞDev use directly `--chain=dev`.
+
+Note2. If the reference machine not support wasmtime, you should replace `--wasm-execution=compiled`
+by `--wasm-execution=interpreted-i-know-what-i-do`.
+
+## What the reference machine is?
+
+For now (06/2022), it's a `Raspberry Pi 4 Model B - 4GB` with an SSD connected via USB3.
+
+To cross-compile the benchmarks binary for armv7:
+
+```
+./scripts/cross-build-arm.sh --features runtime-benchmarks
+```
+
+
+The cross compiled binary is generated here: `target/armv7-unknown-linux-gnueabihf/release/duniter`