[dev] Upstream rebase against Monero v0.18.1.2

[a5no]

This is a dev PR intended to track rebase progress.

Approach taken is based on recommendation from @thaerkh

This rebase is currently setting Monero v0.18.1.2 as our upstream to match against (
https://github.com/monero-project/monero/tree/v0.18.1.2 - commit hash 66184f30859796f3c7c22f9497e41b15b5a4a7c9)

I will be tackling the heavier features in Masari (such as SECOR/uncle mining) after the first milestone of achieving a successful sync until that HF version. As recommended, I will also be minimizing diffs against Monero to support easier upstream updates that we may like in the future.

This draft PR will be converted once it's ready for review. It won't include the hard fork settings that finalize the rebase, and that will be a separate PR in line with what's agreed upon by the community.

[a5no]

2023-03-29 update:

According to consultation, replaced the upstream hardcoded numbers with reasonable global variables (b919a42).

Identified next steps:

• compiling and testing these against the upstream blockchain to ensure validity.
• replacing them with the equivalent for Masari's blockchain
• Add and test changes (such as difficulty algo differences, fixed ring sizes, etc.), working up to uncle mining as a milestone
• Port SECOR / Uncle Mining protocol and test against current chain