fix: replace math.Pow with exact integer constants

[DeVikingMark]

Description

Replaces math.Pow calls with exact integer constants and bit shifts to avoid potential floating-point precision issues when computing powers of 2.

Changes

• Replace math.Pow(2, 32) with constant 4294967296 in NewF() (line 80)
• Replace math.Pow(2, 27) with constant 134217728 in reduceWithMaxBits() (line 390)
• Replace math.Pow(2, 27) with bit shift big.NewInt(1).Lsh(big.NewInt(1), 27) in SplitLimbsHint() (line 465)