apply/lambda Semantics

Nullary metafunction classes are now consistent with all other metafunction classes, rather than being identical to nullary metafunctions:
The apply metafunction now automatically invokes lambda on its first argument. For example, the following is well-formed now:
The lambda implementation for broken compilers has been reworked to emulate the semantics of the full-fledged version as closely as possible; consequently, any code out there that was taking advantage of the semantic quirks of less complete emulation in the previous release may stop working with the new one.

To get a detailed picture of the new apply/lambda semantics, please see the test case at libs/mpl/test/apply.cpp and the corresponding sections of the MPL reference manual.

| [7] | The second form is available only on fully conforming compilers. |