add combine extension for iterables

[iapicca]

• [yak_utils] implemnt and test LetX and LetAsyncX
• [yak_utils] fix Cache
• [yak_utils] fix Cache (1)
• [yak_result] move thenRun extension
• [yak_runner] implement and test letRun and thenRun
• [yak_runner] update pubspec and changelog
• [yak_runner] add forEachRun
• [yak_utils] remove letAsync (it is just 'then')
• [yak_runner] bugfix for VoidResultThenRunX
• [yak_utils] bugfix for LetX
• [yak_result] rework extensions
• [yak_runner] rework extensions
• [yak_runner] fix extension test for thenRun
• bump version in changelog
• update examples
• update dependencies
• [yak_result] add combine extension for iterables