[refac] deprecate emit() and emitrs(), use emit_yaml() and emitrs_yaml()

[biojppm]

fixes #120

[codecov]

Codecov Report

Merging #303 (8654e32) into master (d6147a0) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #303   +/-   ##
=======================================
  Coverage   97.23%   97.23%           
=======================================
  Files          78       78           
  Lines       18358    18358           
=======================================
  Hits        17850    17850           
  Misses        508      508           

Impacted Files	Coverage Δ
samples/quickstart.cpp	99.54% <100.00%> (ø)
src/c4/yml/emit.def.hpp	92.19% <100.00%> (ø)
src/c4/yml/emit.hpp	98.24% <100.00%> (ø)
test/test_basic.cpp	96.55% <100.00%> (ø)
test/test_block_literal.cpp	99.46% <100.00%> (ø)
test/test_case.hpp	44.91% <100.00%> (ø)
test/test_double_quoted.cpp	99.58% <100.00%> (ø)
test/test_emit.cpp	98.15% <100.00%> (ø)
test/test_group.cpp	97.78% <100.00%> (ø)
test/test_group.hpp	100.00% <100.00%> (ø)
... and 15 more

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