Do not rebuild if default_precision would not change

[alamb]

Closes #105, which explains the rationale for this fix

Here is a proposed patch to avoid rebuilding bigdecimal always

I tested locally with

cargo test
cargo test
cargo test
..

With this patch the subsequent calls to cargo test do not force a rebuild

[alamb]

cc @akubera

[akubera]

Oops, yeah that makes sense. I must have thought it checked the contents rather than modtimes.

Here are some suggestions to make it compatible with older versions of Rust.

[codecov-commenter]

Codecov Report

Merging #106 (94e1d42) into trunk (b80509a) will not change coverage.
The diff coverage is n/a.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@           Coverage Diff           @@
##            trunk     #106   +/-   ##
=======================================
  Coverage   75.64%   75.64%           
=======================================
  Files           8        8           
  Lines        2004     2004           
  Branches     2004     2004           
=======================================
  Hits         1516     1516           
  Misses         82       82           
  Partials      406      406