Convert SkSL::to_string to skstd::to_string.

Other than the `float`/`double` overloads, the only value-add of these
methods is to cast the result of `std::to_string` to `SkSL::String`.
When `SkSL::String` is removed, these no-op calls can be replaced with
direct calls to `std::to_string` instead. (The float overloads can
continue to live in `skstd` since their output isn't the same.)

Change-Id: I4df841403114b401ad58017f0264a246fef341af
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/502786
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
20 files changed