feat(stats): add regression and volatility wrappers

- add native TA-Lib bindings in functions.go for beta, correl, linear regression, stddev, tsf, and variance
- register the new TA-Lib symbols in loader.go
- add public wrappers in statistic_functions.go for Beta, Correl, LinearReg, LinearRegAngle, LinearRegIntercept, LinearRegSlope, StdDev, TSF, and Var
- update README.md to document Price Transforms and Statistic Functions

README.md

@@ -37,7 +37,7 @@ This project currently exposes a focused subset of TA-Lib functions:
 - `AvgPrice`, `MedPrice`, `TypPrice`, `WCLPrice`
 
 **Statistic Functions**
-- `AvgDev`
+- `AvgDev`, `Beta`, `Correl`, `LinearReg`, `LinearRegAngle`, `LinearRegIntercept`, `LinearRegSlope`, `StdDev`, `TSF`, `Var`
 
 ## Requirements