Adds slice utils and val.Default()

[aidenwallis]

utils

• utils.ValueAtIndex to fetch the value (or default) value at the index of a given slice. This is panic safe, and will return a bool as the second return value to indicate whether the value existed in the slice.
• utils.IndexInSlice returns a boolean that indicates whether a value exists at a given index in a slice.

val

• val.Default will return the default zero value for any given type parameter. This is just syntactical sugar on top of doing var v T