Refuse numbers a double cannot hold; exact powers up to the limit (closes #3)

A power computed in float64 is refused unless its base and its result
are normal doubles, and so is a result other than zero that is not one:
below about 2.2e-308 a double keeps fewer digits, and at 0 or infinity
none. A literal that go/constant reads as 0 though it is not, such as
1e-999999999, is refused too.

A whole exponent up to 4096 either way is computed exactly and kept
when go/constant holds the result exactly, so 2^-1400 is exact.

ErrTooLarge becomes ErrOutOfRange, and its reply, "That needs a number
too large or too small for me.", is true of both.

Model: opus-5-5

internal/bot/bot_test.go

@@ -27,7 +27,8 @@ func TestReply(t *testing.T) {
 	for in, want := range map[string]string{
 		"hello":    "I only understand arithmetic",
 		"1 / 0":    "I cannot divide by zero.",
-		"1e400":    "The result is too large for me.",
+		"1e400":    "That needs a number too large or too small for me.",
+		"1e-1300":  "That needs a number too large or too small for me.",
 		"(-8)^0.5": "A negative number to a fractional power has no real",
 		strings.Repeat("1+", calc.MaxInputLength) + "1": "That is too long for me",
 	} {