Implement load_average, CPU's frequency and vendor_id

[GuillaumeGomez]

Fixes #215

Taking over #244.

@nrc: couldn't push to your branch so continuing here.

[GuillaumeGomez]

@nrc: About your new net.rs file: why not extending the network.rs existing file?

[GuillaumeGomez]

Ok, I think we're good enough here, this PR is already way too big. What remains to be done:

• Provide network API to get packets and errors (Extend network #250)
• Check if the disk and components APIs can be used like the network one (adding iterator and mutable access to give more control to users)
• Fix clippy lints

[nrc]

Nice! Thanks for pushing this! Sorry I didn't have time to look it over