feat(analysis/special_functions/complex/arg): relation to `real.angle.to_real` (#16205)
feat(analysis/special_functions/complex/arg): relation to `real.angle.to_real` (#16205)

Add two lemmas about the fact that `real.angle.to_real` is the inverse
of coercing the result of `complex.arg` to `real.angle`.  Thus, give
the existing lemma `arg_coe_angle_eq_iff` a much shorter proof.







leanprover-community/mathlib,leanprover-community/mathlib