SetStdio function enables redirection of stdin, stdout, and stderr.

summary of diff --git  a/main.go b/main.go

- Add `io` to import secondary list in `main.go`
- Remove the short forms `Pl` and `Pf` from the variable list
- Add shortcuts for `Stdin`, `Stdout`, and `Stderr` in variable list
- Implement `SetStdio` function to allow redirection of stdin, stdout, and stderr
- Redefine `Pl` and `Pf` as functions that allow redirection of stdout when `Stdout` is set to a desired `io.Writer`

fix Tassert argument formatting

fix Tassert argument formatting

fix Tassert argument formatting

add Pl() etc. shortcuts to README

add ck-return example, expand README a little more

Add Spprint(), upgrade go.mod to 1.17

- 1.17 may not be required, but we know it needs to be newer than 1.13

export AdaptErr

refactor Tassert to use FormatArgs

add Fpf and Pprint, export FormatArgs