= add enhanceSchema function that can add/replace types/directives, see detailed comment with regard to motivation even for simply adding a new type (vs just using toConfig)

= simplify field helper functions

more complex modifyFields function is not necessary, empty types can be added and later extended so if multiples types with cross-referencing fields need to be added, the types can be added first without fields, and the fields appended later.

= appendFields currently adds fields to a new type if no existing type is found, this functionality perhaps should be made configurable or put in a separate function

… mapping

this is necessary to eventually replace use of healSchema and forEachDefaultValue within AddResolversToSchema, although use of mapSchema will require that function to be immutable

= appendObjectFields and removeObjectFields better describe that these functions only apply to fields of GraphQLObjectType

= enhanceSchema is a bit vague

All input types mapped to input object types!

This change:
= bring default value updating into mapSchema

= change aaddResolversToSchema,
addCatchUndefinedToSchema, addErrorLoggingToSchema,
addSchemaLevelResolver to all use mapSchema to return a new schema without modifying the passed in schema

= change all internal use of these functions and all tests to use the return value

= pass typeName instead of a type to field mappers, as passing a type can be confusing, because the type may be in the process of being mapped and not equivalent to the type in the original schema

** the visitSchema function and the schema directive classes that use visitSchema are the next (final?) target for mutable => immutable conversion

there are only really two types of fields, "composite fields" or "output fields" called "fields" in graphql-js on object and interfaces, and input fields on input objects.

The generic FieldMapper type is useful for internal function mapFields so that field mapping logic does not have to be duplicated between output and input fields because of the different types.