Change name from fast/safe to something else

[mdedetrich]

@Ichoran (and others from the discussion at scala/slip#28 (comment)) have a fairly valid point that the current names for safe/fast can be confusing

He suggested something like strict/permissive, from

@mdedetrich - I'd suggest "strict" and ("lax" or "permissive"). That gives you a better idea of what the use cases are. "mapped" and "raw" would be my second choices. Maybe you can come up with others, but I think the existing names have demonstrated themselves as not being good enough. It'd be better to name them sqyrg and ziskob than what they're lableled now, as then people would be forced to read the docs to figure out what the distinctions are.