MathUtil: C++20/23 modernisations

JitCache has been using a class called ValidBlockBitSet. This commit
replaces it with a new class called LazyMemoryRegionBitSet, which works
similarly but is backed by a LazyMemoryRegion, letting us skip
allocating memory for parts of the bit set that correspond to emulated
memory regions where the game isn't storing any code. I have also added
optimized functions for setting or clearing long runs of bits.

Additionally, this commit replaces JitBase's three std::unordered_sets
that are keeping track of instructions with LazyMemoryRegionBitSets.
Here the benefit is more speed than memory usage. This improves NBA Live
2005's menu performance by 1% or so in SuperSamus's testing.

I've also created DegreesToRadians/RadiansToDegrees utility functions since there was a lot of repeated code

StringUtil: Make UTF16ToUTF8 and UTF8ToUTF16 use custom encoding/decoding implementation.

Accomplished using `run-clang-tidy` with `performance-move-const-arg,performance-unnecessary-value-param,modernize-pass-by-value`.

Changed arguments to const references, removed them where inappropriate (e.g. sink parameters). Same with std::move.

Manually reviewed each change to make sure that it makes sense, and do something more appropriate if possible.