New APIs for safe numeric values

[knz]

Fixes #16.
cc @jbowens as this would have been useful in Pebble.

This introduces the following.

New basic types:

// SafeInt represents an integer that is not a sensitive value.
type SafeInt

// SafeUint represents an integer that is not a sensitive value.
type SafeUint

// SafeFloat represents a floating-point value that is not a sensitive value.
type SafeFloat

New methods in SafeWriter:

type SafeWriter interface {
    ...
    // SafeInt emits a safe integer.
    SafeInt(SafeInt)

    // SafeUint emits a safe unsigned integer.
    SafeUint(SafeUint)

    // SafeFloat emits a safe floating-point value.
    SafeFloat(SafeFloat)
}

---

This change is