efficient renamed copy for partials

[ichiban]

Renamed copy of engine.partial was a bit tricky when it's a cyclic term and it used to fallback to *engine.compound.

This PR modifies engine.partial a bit so that we can get an efficient renamed copy of *engine.partial.

$ $(go env GOPATH)/bin/1pl inriasuite.pl
Top level for ichiban/prolog v0.14.0
This is for testing purposes only!
See https://github.com/ichiban/prolog for more details.
Type Ctrl-C or 'halt.' to exit.
?- copy_term([a, b|X], C), go_string(C, S).
X = _409,
C = [a,b|_420],
S = '&engine.compound{functor:".", args:[]engine.Term{"a", &engine.compound{functor:".", args:[]engine.Term{"b", 420}}}}'

$ go run cmd/1pl/main.go cmd/1pl/interpreter.go 
Top level for ichiban/prolog 
This is for testing purposes only!
See https://github.com/ichiban/prolog for more details.
Type Ctrl-C or 'halt.' to exit.
?- copy_term([a, b|X], C), go_string(C, S).
X = _270,
C = [a,b|_281],
S = 'engine.partial{Compound:engine.list{"a", "b"}, tail:281}'

[codecov]

Codecov Report

Base: 99.06% // Head: 99.06% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (550c0c3) compared to base (16f5086).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #268   +/-   ##
=======================================
  Coverage   99.06%   99.06%           
=======================================
  Files          22       22           
  Lines        7148     7162   +14     
=======================================
+ Hits         7081     7095   +14     
  Misses         50       50           
  Partials       17       17           

Impacted Files	Coverage Δ
engine/builtin.go	99.09% <100.00%> (+<0.01%)	⬆️
engine/clause.go	98.40% <100.00%> (ø)
engine/compound.go	100.00% <100.00%> (ø)
engine/env.go	98.93% <100.00%> (+0.01%)	⬆️

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