Math Postfix expressions on numbers

[ogoffart]

We should support for postfix (memeber) functions on number for math expressions:

So we could write things like

foo.clamp(0, 1)
foo.max(bar)
foo.floor()
foo.round()
foo.abs()
foo.sin()

[ogoffart]

In order to implement this, we need to add lookup to the integer types in this trait implementation:

slint/internal/compiler/lookup.rs

Line 917 in b80f455

impl LookupObject for Expression {

Then add a NumberExpression similar to StringExpression in that file.

[WilstonOreo]

This looks useful, especially for the clamp function!
The clamp function, like min/max could support other types than float/int though, e.g. a duration or length.

Btw: I really like how things in lookup.rs are implemented!