Infix and prefix operator definition parsing

[cvauclair]

Description

The Mesh Language should support the definition of custom infix and prefix operators using a let binding with a special syntax.

Infix operators should be defined using the following syntax:

let (OP) = (a, b) => // body

Example:

let (+) = (a, b) => add(a, b)
let (%&%) = (a, b) => a

Prefix operators should be defined using the following syntax:

let (!OP) = (a) => // body

Examples:

let (!$) = (a) => price_feed(a)