💥 Implement wad_ln and wad_exp Functions & wad_cbrt Refactor

[pcaversaccio]

🕓 CHANGELOG

Closes #92. This PR introduces two new wad functions:

• wad_ln: Calculates the natural logarithm of a signed integer with a precision of 1e18. Note that it returns 0 if given 0. Furthermore, this function consumes about 1,400 to 1,650 gas units depending on the value of x.
• wad_exp: Calculates the natural exponential function of a signed integer with a precision of 1e18. Note that this function consumes about 810 gas units.

♻️ Refactoring

• Due to gas efficiency reasons, we directly use 🐍 snekmate's log_2 function in the internal calculation of wad_cbrt.
• We fix a typo in the NatSpec @return comment of the function int256_average.
• We change the logical sequence order of the math functions (including the test functions sequence).
• We add additional version-based links to the CHANGELOG entries.

🔁 Dependencies

• We add solmate as a submodule.
• We add solady as a submodule.
• We bump all submodules to the latest available commits.

🐶 Cute Animal Picture

[pcaversaccio]

LGTM :) - merging after all fuzz tests have passed 🥳