Add links to stdlib in language tour where applicable

[John Björk]

Some chapters in the book about types have obvious counterparts in the stdlib. It would be nice to provide a link for diving deeper into how the stdlib can be utilised when working with these types.

For example the chapter on Result already have a paragraph mentioning the stdlib so it would be easy to add something like this:

The standard library gleam/result module contains helpful functions for
working with the Result type, make good use of them!

Stdlib references

• gleam/result

The exact format can be discussed but if would be nice if it was somewhat consistent through the book.

Some other chapters that could get a link to stdlib are:

• Bit String
• Bool
• Functions
• Ints and floats
• Lists (maybe link to iterator and set also)
• Strings

[Louis Pilfold]

Sounds good!