v0.1.0

First release: Dec, a fixed-scale decimal backed by an i128 with
Dec::SCALE decimal places, with parsing, formatting, arithmetic, rounding,
and optional serde and rust_decimal support.

Arithmetic is exact over a symmetric finite range of roughly +/-1.7e20.
Anything leaving it becomes Dec::NAN, which propagates through every later
operation so the fault reaches the boundary where Dec::is_finite is checked,
rather than being clamped to a plausible number or raised as a panic on a hot
path. The ordering stays total, so Dec remains usable as a map key and in a
sorted book. See the design module for the reasoning.