Update the custom hint example (#1258)

* Add custom_hint_example to workspace, remove local Cargo.lock, fix example

* updated the `CHANGELOG`

* move custom_hint example to examples folder, move this fix to upcoming changes, define example inside vm package

* fix makefile, add std flag for example main

* fix no_std behaviour on example

* change required feature std to example definition

* remove unecessary allow unused_imports

---------

Co-authored-by: Tomás <47506558+MegaRedHand@users.noreply.github.com>
Co-authored-by: Pedro Fontana <fontana.pedro93@gmail.com>

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 #### Upcoming Changes
 
+* fix: updated the `custom_hint_example` and added it to the workspace [#1258](https://github.com/lambdaclass/cairo-rs/pull/1258)
+
 * Add path to cairo-vm README.md [#1276](https://github.com/lambdaclass/cairo-rs/pull/1276)
 
 * fix: change error returned when subtracting two `MaybeRelocatable`s to better reflect the cause [#1271](https://github.com/lambdaclass/cairo-rs/pull/1271)