fix: Incorrect scale used in log and exp for Decimal type

[lukemanley]

from decimal import Decimal as D
import polars as pl

df = pl.Series("a", [D("1.0")]).to_frame()

df.with_columns(
    pl.col("a").log().alias("a_log"),
    pl.col("a").exp().alias("a_exp"),
)

BEFORE:

┌──────────────┬──────────┬──────────────┐
│ a            ┆ a_log    ┆ a_exp        │
│ ---          ┆ ---      ┆ ---          │
│ decimal[*,1] ┆ f64      ┆ f64          │
╞══════════════╪══════════╪══════════════╡
│ 1.0          ┆ 2.302585 ┆ 22026.465795 │
└──────────────┴──────────┴──────────────┘

AFTER:

┌──────────────┬───────┬──────────┐
│ a            ┆ a_log ┆ a_exp    │
│ ---          ┆ ---   ┆ ---      │
│ decimal[*,1] ┆ f64   ┆ f64      │
╞══════════════╪═══════╪══════════╡
│ 1.0          ┆ 0.0   ┆ 2.718282 │
└──────────────┴───────┴──────────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.73%. Comparing base (7ccb3ae) to head (6bb7135).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20888   +/-   ##
=======================================
  Coverage   79.73%   79.73%           
=======================================
  Files        1566     1566           
  Lines      222591   222606   +15     
  Branches     2572     2572           
=======================================
+ Hits       177473   177496   +23     
+ Misses      44526    44518    -8     
  Partials      592      592           

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[orlp]

Thanks!