Progress through Rust exercises

Exercise	DONE ?
intro	✅
variables	✅
functions	✅
if	✅
primitive_types	✅
vecs	✅
move_semantics	✅
structs	✅
enums	✅
strings	✅
modules	✅
hashmaps	✅
options	✅
error_handling	✅
generics	✅
traits	✅
lifetimes	✅
tests	✅
iterators	✅
smart_pointers	✅
threads	✅
macros	✅
clippy	✅
conversions	✅