Tasks
 - [x] finish the 'simple' arena implementation.
   - [x] arena_init
   - [x] arena_alloc
   - [x] arena_release
 - [ ] String8 implementation
   - [x] String8 helpers
   - [ ] String8Node and String8List.
     - [ ] push node
     - [ ] list join
   - [x] Convert cstring to String8.
 - [ ] Read/Write to file.
 - [ ] Create CLI output.
 - [ ] Testcases?