Implement __syntax__ pragmas

[andychu]

important:

• oil-paren -- ( is for expression mode
• oil-brace -- { is an operator
  • related to implement { syntax in for loops #304 somewhat (it doesn't need a pragma)
• oil-set -- take over set keyword for assignment. legacy use is builtin set ...

not sure:

• oil-equals == = for assignment. we already have var and set

[andychu]

Implemented this as shopt -s oil-parse-at. There are too many places where we reuse the parser to scope it to parse time, so I made it a global execution option.

It has the additional restriction of only being executed at the top level.