Expose getElementType and getMapKeyType from MemberDetails

[sebersole]

    /**
     * Get the plural element type for this member.  If the member does not have a type or the
     * member is not plural, a {@code null} is returned.
     * <p/>
     * For arrays, lists and sets the element type is returned.
     * <p/>
     * For maps, the value type is returned.
     */
    TypeDetails getElementType();

    /**
     * Get the map key type for this member.  If the member does not have a type or the
     * member is not a map, a {@code null} is returned.
     */
    TypeDetails getMapKeyType()

Allows to centralize the logic needed to access these generics resolutions.

getMapKeyType is only valid for use with Map typed members. It will simply return null for all others.

• For an array, list and set the element type is the element type.
• For a map, the element type is the value type
• For all others (non plural) the return is null.

[sebersole]

With the introduction of proper generics handling, this is no longer important.

[sebersole]

Reviving this. Nice to cetralize this resolution code.