pkg/math: reuse Ln2 and Ln10 in Log2E and Log10E

Upstream Go's math package has always used these simple divisions,
and now that pkg/gen.go can resolve complex expressions via go/types,
we can use exactly the same expression in our package.

This change is a no-op, which is guaranteed by Example_constants.
The new code is a lot less verbose and easier to understand, too.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I93dbeb26d3b496c724d53c95afad01f8583e56e0
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/557323
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

pkg/math/math.go

@@ -151,9 +151,9 @@ const (
 	SqrtPhi = 1.27201964951406896425242246173749149171560804184009624861664038 // https://oeis.org/A139339
 
 	Ln2    = 0.693147180559945309417232121458176568075500134360255254120680009 // https://oeis.org/A002162
-	Log2E  = 1000000000000000000000000000000000000000000000000000000000000000.0 / 693147180559945309417232121458176568075500134360255254120680009
+	Log2E  = 1 / Ln2
 	Ln10   = 2.30258509299404568401799145468436420760110148862877297603332790 // https://oeis.org/A002392
-	Log10E = 10000000000000000000000000000000000000000000000000000000000000.0 / 23025850929940456840179914546843642076011014886287729760333279
+	Log10E = 1 / Ln10
 )
 
 // Copysign returns a value with the magnitude