[FEATURE] `while` implementation

[kurtlawrence]

Description

An implementation of while, repeating an expression while a predicate remains true.

What is the input into the expression?

• Is it the block's input? This would make sense for initialisation.
• Or is it like fold which takes a seed value.
• Possibly accept both arity.

How is the return value treated?

• Is it much like fold where input->output->input, etc

Implementing while also opens up the possibility of endless loops. Does ogma provide support to detect this and error on runtime?