Support some string escape seqs in Scheme lexer.

Interpret the following escape sequences in string literals:

\a - alarm (ascii 7)
\b - backspace (ascii 8)
\t - tab (ascii 9)
\n - linefeed (ascii 10)
\v - vertical tab (ascii 11)
\f - formfeed (ascii 12)
\r - return (ascii 13)
\e - escape (ascii 27)
\' - quote
\<newline> - empty string (allows single-line strings to span multiple
  lines.