Reorganize math package into subpackages

- Create math/big for big.Int utilities (HexOrDecimal256, U256, parsing)
- Create math/safe for overflow-safe arithmetic (SafeAdd, SafeMul, etc.)
- Move averager files to root package level
- Add re-exports in root package for backwards compatibility
- Update README with new package structure documentation
- Remove old math/meter directory (consolidated elsewhere)
- Remove redundant math/ subdirectory structure

averager.go

@@ -0,0 +1,16 @@
+// Copyright (C) 2019-2024, Lux Industries Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package math
+
+import "time"
+
+// Averager tracks a continuous time exponential moving average of the provided
+// values.
+type Averager interface {
+	// Observe the value at the given time
+	Observe(value float64, currentTime time.Time)
+
+	// Read returns the average of the provided values.
+	Read() float64
+}