Refactor FunctionCodegen.genJetAnnotations()

Always map descriptor to the callable method and take the signature from there,
since this way the signature appears to be more accurate. The whole
"mapSignature vs mapToCallableMethod.getSignature" story needs some
investigation though.

Also make the method non-static, remove useless checks, etc
Get rid of FunctionCodegen.MethodInfo

generateMethodBody() is now responsible for generating the local variable
table, therefore the data class MethodInfo is not needed anymore to return
multiple values from it
Remove useless methods from codegen utils
Remove useless parameter from FunctionGenerationStrategy