Mathlib 2024 reuses still met today: mul_div_cancel, Int.cast_ofNat, Tendsto.exp

Checked on Mathlib v4.35.0; the dates come from the git history. (1) In 2024-03 (#11530), the field lemmas got ₀ names and the bare names were reused for groups. Old mul_div_cancel a h : a * b / b = a now fails with 'failed to synthesize CommGroup ℝ' and 'argument h has type b ≠ 0 but is expected to have type ℝ'. Use mul_div_cancel_right₀ a h. Likewise div_mul_cancel a h → div_mul_cancel₀ a h and mul_div_cancel_left b h → mul_div_cancel_left₀ b h. The bare mul_div_cancel is now a * (b / a) = b in a CommGroup. (2) Int.cast_ofNat is now about numeric literals (it needs [n.AtLeastTwo]), so the old ((n : ℤ) : R) = n call fails with 'failed to synthesize n.AtLeastTwo'. That statement is now Int.cast_natCast. (3) Since 2024-05 (#12635), Filter.Tendsto.exp is the Banach-algebra version. For f : α → ℝ, hf.exp gives a type mismatch, because its result is about NormedSpace.exp. The real version is hf.rexp. For the ℝ field cases, simp [h] or field_simp also close a * b / b = a without needing the names.