add ability to convert between types consistenly

In our downstream systems and tools (terraform-provider-cloudflare,
cf-terraforming) we often have to swap between Golang types and pseudo
types. One example is with Terraform schemas and our struct types. There
isn't a consistent way which means they are all implemented slightly
different and there isn't a great convention. This commit aims to solve
that by introducing a standard to adhear to by introducing a number of
helper methods.

- <type>Ptr: Accepts a value and returns a pointer.
- <type>Value: Accepts a pointer and returns a value.
- <type>PtrSlice: Accepts a slice of values and returns a slice of pointers.
- <type>ValueSlice: Accepts a slice of pointers and returns a slice of values.
- <type>Map: Accepts a string map of values into a string map of pointers.
- <type>ValueMap: Accepts a string map of pointers into a string map of values.

Example in Terraform where the schema is a string but we want a pointer.

```go
examplePtr := cloudflare.StringPtr(d.Get("example").(string))
```