Fix block validation failure in Windows for Rust tool-chain nightly-2020-06-10

[hansieodendaal]

Description

1. Commit df95cee introduced a bug in Windows builds that resulted in certain blocks failing validation tests. The cause
   was traced to inconsistent implementation of the std::f64::powi function in certain Rust toolchains, where
   the Tari Windows code would give a slightly different floating-point result than Linux. This affected the
   missionSchedule::block_reward calculation, which caused block validation for certain block numbers to fail.
   The solution was to use crate num::pow for the power calculation.
   (See related Rust issue: Implementation of std::f64::powi function gives inconsistent answers in multiple Rust tool-chains rust-lang/rust#73920)

2. Changed two other instances where std::f64::powi were used.

Example,

std::f64::powi(0.9999995604090385, 9182) => 0.9959718099156295 expected, but returns 0.9959718099154268

results in

EmissionSchedule::block_reward(9182) => 5517534590 expected, but returns 5516534589

This closes #2022.

Motivation and Context

When using Rust toolchain nightly-2020-06-10 all block validation tests should pass, otherwise a base node will not be able to sync to the blockchain.

How Has This Been Tested?

Tested in Windows 10 and Linux Ubuntu 18.04.4 LTS .

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.