Use raw executor by default, skip intermediate shell

[sharkdp]

This adds a new "raw executor" (next to the "shell executor") that
allows hyperfine to execute commands directly without any intermediate
shell.

The command line is split into tokens (using the shell-words crate),
and according to POSIX rules. The first token is taken as the executable,
and the rest as arguments.

The new executor is enabled by default. In order to select the shell
executor, users will have to pass --shell=default.

This allows us to reduce measurement noise and to benchmark very quick
commands. It also decreases the time to run benchmarks, as we don't need
the calibration phase.

Also, it allows one to make sure that the executed command is not
implemented as a shell builtin. For example hyperfine true
and hyperfine --shell=default true return different times due
to the fact that bash executes true as a NOP.

Co-authored: Ciprian Dorin Craciun ciprian@volution.ro

To do:

• Add more integration tests.
• Think about whether we really want this as a default. Many use cases would not work out of the box (e.g. hyperfine "fd . ~"). And this could lead to surprising results: hyperfine "echo hi && sleep 0.1" would result in echo 'hi' '&&' 'sleep' '0.1'
• Write CHANGELOG entry and update documentation.
• Update --help text (regarding --shell=default and <command>)