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>

0 files changed, 0 insertions, 0 deletions