Implement `String.concats` and `Bytes.concats` in stdlib

Motivation and Context

As it was reported by Rodrigo in #ligo, there are missing functions for concatenating lists of bytes and strings (using Michelson's CONCAT).

Description

In this MR we implement CONCAT for lists of bytes/strings using an internal op. C_CONCATS.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

New functions have been implemented for concatenating lists of strings and bytes:

• String.concats : string list -> string
• Bytes.concats : bytes list -> bytes

Checklist:

• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).