Specialize TemplatableValue so non-string types use function-pointer-only
storage (4 bytes on 32-bit) instead of the tagged union with std::function
support (8 bytes). The std::string specialization retains full support for
VALUE, STATIC_STRING, FLASH_STRING, and stateful lambdas.
Codegen now wraps non-string constants in stateless lambdas automatically,
so the generated C++ always assigns a function pointer.