Windows block validation tests fix

[hansieodendaal]

Description

Rolled back the rust-toolchain, clear_on_drop and tari_crypto due to certain block validation tests failing in Windows.
These validation tests now pass:

test test_block_33923 ... ok
test test_block_34947 ... ok
test test_block_30335 ... ok
test test_block_9182 ... ok

TODO: Further investigation is needed to fix the cause of the problem, see registered issue #2022.

Motivation and Context

Commit df95cee introduced a bug in Windows builds that resulted in blocks 9182, 30335, 33923 and 34947 failing validation tests. e,g,:

2020-06-18 11:39:51.632946300 [c::bn::comms_interface::inbound_handler] WARN  Block #33923 
   (3a541cf21e19650697d8d785ba9b5f38e86868af94a7fca4adec3c3bcf93aef1) validation failed: ValidationError { source: 
   TransactionError(ValidationError("Sum of inputs and outputs did not equal sum of kernels with fees")) }

This originates from fn validate_kernel_sum(...) in aggregated_body.rs, as fn sum_kernels(...) returns the wrong result.
The problem is caused by updating Rust toolchain to nightly-2020-06-10 from nightly-2020-01-08 and clear_on_drop to 0.2.4 from 0.2.3.

How Has This Been Tested?

• Tested with a tari_base_node.exe in Windows, starting the blockchain from the last known good state at heights 9181, 30334, 33922 and 34946.
• Unit tests in base_layer\core\tests\regression_tests.rs also passed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Feature refactor (No new feature or functional changes, but performance or technical debt improvements)
• New Tests
• Documentation

Checklist:

• I'm merging against the development branch.
• I ran cargo-fmt --all before pushing.
• I have squashed my commits into a single commit.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[hansieodendaal]

Closed in favour of #2035. The branch should not be deleted if the tagged version v0.4.2-win for Windows builds needs to be kept.