strconv: clean up powers-of-10 table, tests

Both Eisel-Lemire and Ryu depend on a table of
truncated 128-bit mantissas of powers of 10,
and so will Dragonbox.

This CL:
 - Moves the table to a separate file, so it doesn't look tied to Eisel-Lemire.
 - Introduces a uint128 type in math.go for the table values,
   since .Hi and .Lo are clearer than [1] and [0].
 - Generates the table from a standalone generator pow10gen.go.
 - Adds a new pow10 function in math.go to handle table access details.
 - Factors a 64x128->192-bit multiply into umul192 in math.go.
 - Moves multiplication by log₁₀ 2 and log₂ 10 into math.go.
 - Introduces an import_test.go to avoid having to type differently
   cased names in test code versus regular code.
 - Introduces named constants for the floating-point size parameters.
   Previously these were only in the floatInfo global variables.
 - Changes the BenchmarkAppendUintVarlen subtest names
   to be more useful.

Change-Id: I9826ee5f41c5c19be3b6a7c3c5f277ec6c23b39a
Reviewed-on: https://go-review.googlesource.com/c/go/+/712661
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/strconv/export_test.go

@@ -4,7 +4,27 @@
 
 package strconv
 
+type Uint128 = uint128
+
 var (
-	BitSizeError = bitSizeError
-	BaseError    = baseError
+	BaseError        = baseError
+	BitSizeError     = bitSizeError
+	MulLog10_2       = mulLog10_2
+	MulLog2_10       = mulLog2_10
+	ParseFloatPrefix = parseFloatPrefix
+	Pow10            = pow10
+	Umul128          = umul128
+	Umul192          = umul192
 )
+
+func NewDecimal(i uint64) *decimal {
+	d := new(decimal)
+	d.Assign(i)
+	return d
+}
+
+func SetOptimize(b bool) bool {
+	old := optimize
+	optimize = b
+	return old
+}