Fully support importing previous symbol/macro table context

[tgregg]

Currently, the encoding directive readers support simple appends to the previous context using $ion_encoding as the first value following (macro_table or (symbol_table. However, the following is also legal, and should be supported.

$ion_encoding::(
  (symbol_table [a, b, c] $ion_encoding [g, h, i])
  (macro_table (macro pi () 3.14159) $ion_encoding)
)