Use the term "signature" instead of typings when refering to methods

[BelfordZ]

https://github.com/open-rpc/typings/blob/master/src/typescript.ts#L13

[BelfordZ]

as discussed:
generator interface should export:
getMethodTypings -> same thing + method mapping
getMethodSignatures -> get only method sigs
getMethodSignature -> get only method sig (one of them)
getMethodMapping -> returns the method mapping (name of method -> method signature)