Add as_u256 for U128

[SwayStar123]

Description

Adds the conversion helper for U128

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
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[IGI-111]

Not against having an own method for this, but why not just implement From<U128>?

[SwayStar123]

Not against having an own method for this, but why not just implement From<U128>?

sway/sway-lib-std/src/primitive_conversions/u256.sw

Line 163 in a879ffb

impl From<U128> for u256 {

We have that already. This is just to ensure consistency as we have as_x functions for all other upcasts

[IGI-111]

I see, makes sense.