Pointer reference to nature

[antonmedv]

@diegommm We now use *nature.Nature and nature.Nature inside.

Should we switch to only using *nature.Nature?

expr/ast/node.go

Lines 34 to 37 in bcb2d51

	type base struct {
	loc file.Location
	nature nature.Nature
	}

expr/ast/node.go

Line 18 in bcb2d51

Nature() nature.Nature

[antonmedv]

Otherwise this makes it difficult to use:

[diegommm]

I will look into this.

[diegommm]

Fixed in #849