LET-CONST support should be added

[milesrout]

Support for LET-CONST in the form of a syntactic form:

(let-const ((<var> <expr>) ...) . <exprs>)

The only difference between let-const and let should be let-const should create environments with ConstCells instead of Cells, and ConstCells should not be able to be mutated through SET! (or anything else).

{ 'a': Cell(nil) }      + { 'a': Cell(Symbol('a')) }      = { 'a': Cell(nil) } # current behaviour
{ 'a': ConstCell(nil) } + { 'a': Cell(Symbol('a')) }      = ... # ?
{ 'a': Cell(nil) }      + { 'a': ConstCell(Symbol('a')) } = ... # ?
{ 'a': ConstCell(nil) } + { 'a': ConstCell(Symbol('a')) } = { 'a': ConstCell(nil) } # obvious