Util: add StringData, String, SliceData(go 1.20)

[monkey92t]

See golang/go#53003

As stated in Proposal:

• converting []byte to string
• converting string to []byte
• grabbing the Data pointer field for ffi or some other niche use
• converting a slice of one type to a slice of another type

The first use case can also commonly be seen as *(*string)(unsafe.Pointer(&mySlice)), which is never actually officially documented anywhere as something that can be relied upon. Under the hood, the shape of a string is less than a slice, so this seems valid per unsafe rule (1), but this is all relying on undocumented behavior. The second use case is commonly seen as ([]byte)(unsafe.Pointer(&string)), which is by-default broken because the Cap field can be past the end of a page boundary (example here, in widely used code) -- this violates unsafe rule (1).

:)

[SoulPancake]

Will this be done part by part or all at once?