More number types

[emil14]

At the moment Neva only has int and float, should we support Go's numbers?

int  int8  int16  int32  int64
uint uint8 uint16 uint32 uint64

Upsides

• Better performance (and easy to implement)

Downsides

• User will have to think about number format just like in Go

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This and #28 are both highly coupled with the "lower-level" programming in Neva. We need to be very careful, because message-passing overhead can wipe out all the benefits of having a specific numeric types.