Use consistent collection type in builders for modifiers and type variables.

[GlassBricks]

Right now, modifiers are represented using a MutableSet in TypeAliasSpec.Builder, TypeSpec.Builder, but using aMutableList in FunSpec.Builder, ParameterSpec.Builder, PropertySpec.Builder. It would make always use a Set to match the Specs.

Type Variables are represented using a MutableList in FunSpec.Builder, PropertySpec.Builder, and TypeSpec.Builder, but a MutableSet in TypeAlias.Builder. It would make always use a List (to match the Specs).

This would be backwards incompatible so it will have to wait until 2.0