Data's closure-taking functions should be modernized

[glessard]

The closure-taking functions in Data's API should support

• typed throws
• non-copyable returned values

These are:

• public func withUnsafeBytes<ResultType>(_ body: (UnsafeRawBufferPointer) throws -> ResultType) rethrows -> ResultType
• public func withContiguousStorageIfAvailable<ResultType>(_ body: (_ buffer: UnsafeBufferPointer<UInt8>) throws -> ResultType) rethrows -> ResultType?
• public mutating func withUnsafeMutableBytes<ResultType>(_ body: (UnsafeMutableRawBufferPointer) throws -> ResultType) rethrows -> ResultType

Generalizing these functions on the concrete type does not negatively affect its protocol conformances.