The target beman::execution should always be defined #246

Currently, the CMake target beman::execution is only defined when BEMAN_USE_MODULES is ON. This creates an inconvenience when consuming the library, because one needs to link to beman::execution when BEMAN_USE_MODULES is ON, and to beman::execution_headers otherwise. This shouldn't be the case; the main beman::execution target should always be defined, and should do the right thing in either mode. (That is, it should be equivalent to beman::execution_headers when BEMAN_USE_MODULES is OFF.)