ユーザー関数（Function）を実装しました。

[javecs]

No description provided.

[codecov]

Codecov Report

Merging #52 into master will increase coverage by 0.58%.
The diff coverage is 92.3%.

@@             Coverage Diff              @@
##             master      #52      +/-   ##
============================================
+ Coverage     71.55%   72.13%   +0.58%     
- Complexity       73       80       +7     
============================================
  Files             8        9       +1     
  Lines           450      463      +13     
  Branches         49       52       +3     
============================================
+ Hits            322      334      +12     
  Misses           77       77              
- Partials         51       52       +1

Impacted Files	Coverage Δ	Complexity Δ
src/main/kotlin/xyz/javecs/tools/expr/Function.kt	92.3% <92.3%> (ø)	7 <7> (?)

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