Change generator-rs license

In reddit thread jechse remind me of using MAY project:

A bit of a licensing concern though: the generator dependency
is licensed under the LGPL, and since rust statically links 
dependencies, any project that uses the May crate will also 
be statically linking the LGPL crate, and have to worry about 
all of the licensing implications of that. IANAL, and 
everything may actually be more kosher than I think, but LGPL
code almost always presents a not-insignificant difficulty in 
getting adopted by companies with cautious lawyers in my 

So I decide to change the license of generator-rs from LGPL2.1 to MIT/Apache to remove any obstacle to use the libraries.

I have very little experience of using public license. But I do believe a proper public license could help both the project and users.