Support try expression

[yoshihiro503]

Fix #223
This is a simple implementation of try expression and does not support catch clauses.

• Does not change Ast.t
• All catch clauses are ignored
• Translate try expression to a case expression or a more simple expressions

For the full support of try expression, implements about catch clauses : #252.

simple try expressions

    try
        <exprs>
    catch
        <catch-clauses>
    end.

↓

[<exprs>]

try expressions with pattern clauses (with an of section)

    try
        <exprs>
    of
        <case-clauses>
    catch
        <catch-clauses>
    end.

↓

Case(line, [<exprs>], [<case-clauses>])

try expressions with an after section

    try
        <exprs>
    catch
        <catch-clauses>
    after
        <after>
    end.

↓

Let(line, "_", [<exprs>], [<after>])