fix raw mode output

[ichiban]

While reorganizing code related to streams, I ditched arbitrary io.Reader / io.Writer -backed streams.
It resulted in messed up output in cmd/1pl/main.go since it's in raw mode.

This PR fix the problem by reintroducing arbitrary io.Reader / io.Writer -backed streams.

[codecov-commenter]

Codecov Report

Merging #122 (de6a6d6) into main (4be9111) will increase coverage by 0.28%.
The diff coverage is 89.33%.

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
+ Coverage   85.34%   85.63%   +0.28%     
==========================================
  Files          18       18              
  Lines        5049     5074      +25     
==========================================
+ Hits         4309     4345      +36     
+ Misses        658      648      -10     
+ Partials       82       81       -1     

Impacted Files	Coverage Δ
engine/builtin.go	83.25% <86.44%> (+0.62%)	⬆️
engine/stream.go	100.00% <100.00%> (ø)
interpreter.go	92.22% <100.00%> (ø)

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