Add example for Uppercase and Lowercase ASCII using builtins

[lukewilliamboswell]

See zulip discussion for context

We should add a new example which shows people how to make conversions to/from Upper and Lower case using only the builtins.

We should briefly explain the trade-offs, and how there will be packages available to provide additional functionality. Could even point people towards https://github.com/Hasnep/roc-ascii

Below is a quick and dirty implementation... we should modify this to handle the Result correctly, and polish it a little with some explanation and examples.

module [to_upper, to_lower]

to_upper : Str -> Str
to_upper = \str ->
    str
    |> Str.toUtf8
    |> List.map to_upper_help
    |> Str.fromUtf8
    |> unwrap

to_lower : Str -> Str
to_lower = \str ->
    str
    |> Str.toUtf8
    |> List.map to_lower_help
    |> Str.fromUtf8
    |> unwrap

expect to_upper "hello" == "HELLO"
expect to_upper "Hello" == "HELLO"

expect to_lower "HELLO" == "hello"
expect to_lower "Hello" == "hello"

to_upper_help = \c -> if c >= 'a' && c <= 'z' then c - delta else c
to_lower_help = \c -> if c >= 'A' && c <= 'Z' then c + delta else c
delta = 'a' - 'A'

unwrap : Result a _ -> a
unwrap = \result ->
    when result is
        Ok a -> a
        Err _ -> crash "quick and dirty error handling"