Review which functions should be exported

[mgoerens]

See https://go.dev/ref/spec#Exported_identifiers

An identifier may be exported to permit access to it from another package. An identifier is exported if both:
    1. the first character of the identifier's name is a Unicode upper case letter (Unicode class "Lu"); and
    2. the identifier is declared in the [package block](https://go.dev/ref/spec#Blocks) or it is a [field name](https://go.dev/ref/spec#Struct_types) or [method name](https://go.dev/ref/spec#MethodName).

All other identifiers are not exported.

This might be especially relevant for #47