* Add "std" feature to rustlib to allow using std in Xtensa projects
* Fix cargo target in CMakeLists.txt
* Use std feature when building for Xtensa
* Use std ffi aliases when building with std feature
* Add custom ffi aliases when building without std feature
* Add no_std attribute and panic handler when building without std feature
Juraj Michálek
Robert Cottrell
Ivan Grokhotkov