[dev.regabi] strconv: add to bootstrap packages

go/constant relies on strconv for parsing Go literals, while older
versions of strconv either lack recent Go language features (e.g., Go
1.13's new numeric literals) or have errors (e.g., mishandling of
carriage returns in raw string literals prior to Go 1.8).

This requires two changes:

1. Splitting out the internal/bytealg dependency into a separate file,
which can be easily substituted with a simple loop for bootstrap
builds.

2. Updating eisel_lemire.go to not utilize Go 1.13 functionality
(underscores in numeric literals and signed shift counts).

Change-Id: Ib48a858a03b155eebdcd08d577aec2254337e70e
Reviewed-on: https://go-review.googlesource.com/c/go/+/272749


Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>