Automatically escape type in enum values

[martinbonnin]

Since we can detect, we might as well to it on behalf of the user, this saves some configuration. See #4293

Note that if someone has already done something like this:

extend enum SomeEnum {
  type @targetName(name: "type_")
}

they will have to remove this to keep type_ or else it'll be renamed to type__.

Also remove a workaround we had for 'header' and 'impl' that was adressed in KotlinPoet with square/kotlinpoet#1248

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