Remove core crate feature

[Robbepop]

Closes #270.

In summary this PR does the following:

• Removes the core crate feature. Explanation can be found in Remove core feature #270 .
• Replaces the vec_memory crate feature with the new virtual_memory crate feature.
  • The reason is that the new virtual_memory crate feature is additive unlike the old vec_memory.
  • Users of the crate are recommended to enable the virtual_memory crate feature if they are building wasmi for a platform that supports OS based virtual memory. Otherwise wasmi will fallback onto the vector based implementation.