Improves accuracy and efficiency

Issue #5

e.g.: kprobe:sys_read,sys_write { @[func] = count() }

Issue #2

Issue #1

Behave like regular variables from other languages
- scoped within each probe block
- can only be used after being assigned
- not saved to BPF maps

Codegen: Use unsigned versions of division/modulo operations (LLVM BPF backend doesn't support signed)

Called when '==' or '!=' are used on string values

Allows statements such as '@x = (1 == 2)' to work