feat: continuous compounding

[pakim249CAL]

Fixes #253

The WadMath library from USM is copied over for testing exponentiation. FixedPointMathLib does not come with an e^x wad utility.

[Rubilmax]

The code works, but IMO choosing between $(1+r)^n$ or $e^{nr}$ should only be a business-oriented design choice because the gas difference, complexity and security of the code is almost identical

Pls make sure this design is accepted by @PaulFrambot 🙏

[MerlinEgalite]

The code works, but IMO choosing between (1+r)n or enr should only be a business-oriented design choice because the gas difference, complexity and security of the code is almost identical

Pls make sure this design is accepted by @PaulFrambot 🙏

@PaulFrambot is ok with this