`get_ptr_mut` and useful `From` implementations

• prelude module is removed. Instead pub use orx_pinned_vec::PinnedVec; is added for convenience.
• unsafe get_ptr_mut pinned vec method is implemented. This method has a default Growth implementation; additionally, a more efficient constant time GrowthWithConstantTimeAccess implementation.
• test coverage is improved.
• SplitVec of any Growth, as well as standard Vec, can now be converted into a split vector with Recursive growth with no cost.
• Conversion from SplitVec to Vec is now free provided that the split vector contains only one fragment.
• slice and try_get_slice methods now accept any range.