Promote Option to a compiler-built-in type

[tranzystorekk]

This topic has not seen a good discussion so I've added it here.

Option is considered a fundamental concept in functional-oriented languages, useful in expressing many fallible operations like obtaining elements from maps, representing results of iterator operations etc.
Therefore it seems suitable that they be available in the Gleam prelude, i.e. as a core language type without the need to import them.

The proposal is as follows:

• Make the compiler insert inline Option usage/definitions wherever the type is used, similar to Result
• Remove the Option definition from gleam/option
• Add Option definitions/code to the js prelude
• (Lower priority) Rewrite stdlib APIs that use Result(a, Nil) to Option

Pros:

• Simplified APIs in various places
• Option is more expressive than Result(a, Nil) in most situations
• Less hassle with importing gleam/option, e.g. in stdlib itself

Cons:

• We currently only have try expression support for Result, so some code would have to be rewritten with option.to_result(Nil)

[TanklesXL]

Seconded! This would also unblock what I wanted to do for
gleam-lang/stdlib#214

Only thing I’m not sure I agree on is the replacing usage of Result(a,Nil) but I have no strong opinions on the matter

[tranzystorekk]

A reference implementation I've been working on can be inspected in this branch: https://github.com/tranzystorek-io/gleam/tree/option-as-builtin