fixed shrinking vector and un-mutated primitives

[Knight-Ops]

This fixes an issues with Vec<_> mutation where the mutator can actually try to remove more bytes than exist in the vector. This causes a panic.

The fix here is pretty simple, we just take the min of the length of the vector or the randomly generated range.

[msftclas]

All CLA requirements met.

[Knight-Ops]

New changes include :

Cargo.toml : Adding env_logger and log as dependencies just so you have easier access to trace and debug information

mutator.rs : Added a setter for target_total_fields so that we can actually set the size of the structure instead of setting it always to 1 (Seems like it fell through the cracks). Added a Havoc random field selector, as it stands currently, Havoc will only ever mutate the last field in the structure (for primitive integers, other types of fields are mutated differently).

mutation.rs : Added code generation for setting the actual number of fields of the structure being fuzzed this allows us to actually fuzz each field and not guess later how large the structure is.

EDIT: Removed previous change of targeted_field_idx increment movement

[Knight-Ops]

New changes include:

mutator.rs : Added support for nested structure that are properly mutated with a simple fix of AddAssign the total number of fields to target