Better error messages

[emosenkis]

To get better error messages that just "not found" you could implement the missing methods but have them take an argument whose name is the error message. It's not pretty, but at least somewhere in the "wrong number of args" error message there would be a hint about the actual issue.

[TedDriggs]

Another approach would be to implement the methods and mark them deprecated, then panic at runtime. Not great, but it'd at least get a compile-time message in the right place.

[idanarye]

@TedDriggs - your approach won't work, because if two fields are missing I'll have two deprecated build() methods, and the compiler won't allow that.

I don't think @emosenkis' approach will work either, because I'm not using traits here (don't want to force the user to import a preamble for every TypedBuilder struct they use) and having multiple methods with the same name but different signatures on the same struct (not via traits) is method overloading - which Rust does not support.

[idanarye]

Done in #30 by @seb-bl.