Fix decoding of very small Decimal values from NUMERIC data

[gwynne]

When generating a string representation of PostgresNumeric values, we were misinterpreting the weight value for values with very small fractional parts. This caused decoding Decimals to return values with insufficient leading zeroes. Thanks to @patchthecode for reporting this issue!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.05%. Comparing base (db1eae1) to head (5ed090e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #603      +/-   ##
==========================================
- Coverage   75.11%   75.05%   -0.06%     
==========================================
  Files         132      132              
  Lines        9532     9534       +2     
==========================================
- Hits         7160     7156       -4     
- Misses       2372     2378       +6     

Files with missing lines	Coverage Δ
...ources/PostgresNIO/Data/PostgresData+Numeric.swift	86.48% <100.00%> (+0.14%)	⬆️

... and 4 files with indirect coverage changes

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