spiritd, bug fix.

[Richard Webb]

fwiw, i tried to work around this by changing the second conv function to:

      T conv(T, U)(in Match!(U) arg) { return T(arg._len); }

and then got a bunch of errors like:

    spiritd\impl\parse.d(36): Error: variable m cannot be read at compile time

Which looks like bug 3809.
If i change parseImpl to initialize 't' manually rather than using the struct
initializer, then the test app compiles and appears to work ok.