Add mapValue

[hirataqdees]

This commit adds a function mapValue for mapping a function over values of a given map

[robin-aws]

Just a few suggestions:

Suggested change

	function method {:opaque} mapValue<K, V, V'>(f: V -> V', m: map<K, V>): (m':map<K, V'>)
	function method {:opaque} MapValues<K, V, V'>(f: V ~> V', m: map<K, V>): (m': map<K, V'>)

• Pascal casing since that's what the style guide says:https://github.com/dafny-lang/libraries/blob/master/STYLE.md#naming-convention :)
• You're already accounting for partial functions in your requires, but since you're declaring f to have type V -> V', that means f is a total function and its precondition is always just "true". V --> V' is the arrow type that allows for other non-trivial preconditions, and then V ~> V' is the even more general arrow type that allows for reads clauses as well. You'll have to add a reads clause to the function itself of course, something like reads set o | exists k in m.Keys :: o in f.reads(k).