Missing features:

- automatically print variables on exit
- if/else
- scratch variable ($name)
- `args', tracepoint arguments support (requires kernel work)
- str()
- time()
- @ = sum(x)
- @ = hist(x)
- @ = lhist(x, min, max, step)
- @ = min(x)
- @ = max(x)
- delete(@map[key])

Improvements:

- implement arithmetic operator precedence
- rewrite stmt_printf() to not use strings internally
- look at collision in map hash