Add typed Map[K, V] with map literals and basic methods

[synapticvoid]

Task

Implement Map[K, V] as a built-in generic collection type.

Acceptance criteria

• Type syntax: Map[Str, Int], Map[Str, Bool], etc.
• Non-empty literal infers types: {"a": 1} → Map[Str, Int]
• Empty literal requires annotation: var m: Map[Str, Int] = {}
• Mixed-type literals are a compile-time error
• K must be hashable — compile-time error if not
• Map is unordered; no iteration order guaranteed
• Methods: len(), contains(key), get(key), get(key, default), add(key, value), remove(key)
• Mutating methods (add, remove) require var receiver — compile-time error on const

Spec

See PDP-009: Collection Types.