Add assignment operations

[c3d]

In order to be able to document the built-in equations in a way that is usable in practice, we need to have a quick way to describe a number of variables.

A possible approach is to have an assignment object that is similar internally to unit objects, but with a = separator and a name on the left. This would work like the ▶ operator on the HP50G.

Proposed syntax:

A=50_m
B='30_cm+25_mm'

Note that the syntax cannot be accepted inside algebraic expressions, but it could be rendered as:

50_m▶A
'(30_cm+25_mm)▶B'

This behaves like STO but returns the input value on the stack.

[Wiljea]

So contrary to STO it doesn't consume the value on level 1 of the stack, isn't it ?

[c3d]

Separate assignment operation and assignment object.

The assignment object also tags the result.