Implement `remove_*`, `pop_*`, and `clear` functions

[hrszpuk]

• [x] remove (done but needs fixing, see #21)
• [x] remove_key
• [x] remove_section
• [x] pop_section
• [x] pop_key
• [x] clear

[hrszpuk]

Ah, I added this but forgot I need to free the memory when I remove it...

[hrszpuk]

remove handles both keys and sections. remove_key and remove_section aren't necessary.