Make formatter to print names in memory instead of entire expression (when possible)

[jmansilla]

Steps to reproduce actual behaviour:

λh> TRUE = (λx.(λy.x))
new expression parsed: (λx.(λy.x))
λh> NOT = (λb.(λx.(λy.((b y) x))))
new expression parsed: (λb.(λx.(λy.((b y) x))))
λh> not_true = NOT TRUE
new expression parsed: ((λb.(λx.(λy.((b y) x)))) (λx.(λy.x)))

Desired behavior

λh> not_true = NOT TRUE
new expression parsed: (NOT TRUE)

a) Should be configurable, so the user can easily op-out
b) When enabled, shall be used when printing evaluation/reduction trees

It may be trickier than what it seems, so we may never merge this.