Implement expressions (EXPRSXP)

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Test cases:

• expression(1+2):

  one-element list of

  language(symbol `+`, double(1), double(2))
  

• expression(u, v, 1+ 0:9):

  three-element list of

  symbol `u`, symbol `v`, language(symbol `+`, double(1), language(symbol `:`, double(0), double(9)))
  

• expression(NULL):

  one-element list of

  NULL
  

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In QAP, EXPRSXPs are represented by type XT_VECTOR_EXP, which is encoded the same way as generic lists (XT_VECTOR).