re-add Typedefs.Strings without breaking CI

[andrevidela]

https://github.com/typedefs/typedefs/blob/fb909be7589509df0b78b29d43e35673f5177514/src/Typedefs/Strings.idr

This file has been removed and the functions have been placed in Typedefs.Typedefs. This has been done to satisfy nix to build typedefs, indeed, with the file and the imports present it would fail with the following error:

Type checking ./Typedefs/TermParse.idr
 Can't find import Typedefs/Strings
builder for '/nix/store/z8s5px057m2gamxp9drh87741k3qihzv-idris-typedefs-core-dev.drv' failed with exit code 1
cannot build derivation '/nix/store/2r9jxqszndna5nyvmnqwbbymcnli3h74-idris-typedefs-examples-dev.drv': 1 dependencies couldn't be built
error: build of '/nix/store/2r9jxqszndna5nyvmnqwbbymcnli3h74-idris-typedefs-examples-dev.drv' failed

Those string-specific functions should return to Typedefs.Strings instead of polluting the Typedefs.Typedefs module.