refactor: move amm math into its own crate

[dandanlen]

Moves the amm math primitives into their own crate.

This allows sharing them more easily without depending on the amm (For example, this is why Price was added to cf-primitives: I moved it back.).

I also refactored some little things I noticed while making these changes (see the second two commits).

[codecov]

Codecov Report

Attention: Patch coverage is 88.15490% with 52 lines in your changes missing coverage. Please review.

Project coverage is 72%. Comparing base (755c6eb) to head (340157e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
state-chain/pallets/cf-pools/src/lib.rs	28%	22 Missing and 1 partial ⚠️
state-chain/runtime/src/lib.rs	0%	0 Missing and 19 partials ⚠️
state-chain/amm-math/src/lib.rs	97%	9 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #5418    +/-   ##
======================================
- Coverage     72%     72%    -0%     
======================================
  Files        489     490     +1     
  Lines      86863   86759   -104     
  Branches   86863   86759   -104     
======================================
- Hits       62401   62211   -190     
- Misses     21542   21608    +66     
- Partials    2920    2940    +20     

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