Allow member to inherit from more than one mixin

[sugmanue]

Description of changes:

Allow multiple mixins to override the same member if they all target the same shape. This will allow the following example from the docs

@mixin
structure A1 {
    @private
    a: String
}

@mixin
structure A2 {
    @required
    a: String
}

structure Valid with [A1, A2] {}

To actually work as documented. This will also bring parity to the way mixins behave for top-level shapes, it will allow multiple mixins to override the same member provided that they all agree on the same target and will override traits with a last introduced wins policy.

Traits attached to each member will be overwritten by the last mixin or by the corresponding member in the shape. For instance, the following model

@mixin
structure A1  {
    @tags(["a"])
    a: String
}

@mixin
structure A2  {
    @tags(["b"])
    a: String
}

@mixin
structure Valid with [A1 A2]  {
    @tags(["c"])
    a: String
}

Will be flattened to the following shape:

structure Valid with [A1 A2]  {
    @tags(["c"])
    a: String
}

Notice that the tags trait in the Valid structure will supersede the tags introduced by the mixin members.

And the following model:

@mixin
structure A1  {
    @tags(["a"])
    a: String
}

@mixin
structure A2  {
    @tags(["b"])
    a: String
}

@mixin
structure Valid with [A1 A2]  {
    a: String
}

Will be flattened to the following shape:

structure Valid with [A1 A2]  {
    @tags(["b"])
    a: String
}

Overriding the @tags in the order the mixins are processed, last wins.

This change also forbids following the docs overriding using a different target, for instance

$version: "2.0"

namespace com.amazon.example

@mixin
structure A1 {
    a: Integer
}

@mixin
structure A2 {
    a: String
}

structure Valid with [A1 A2] {
    a: Integer
    c: Integer
}

Will emit a validation error like:

Member conflicts with an inherited mixin member: `com.amazon.example#A2$a`

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mtdowling]

list and map should override this too I think

[sugmanue]

Totally, let me update it.