Fix math spacing around parentheses

[sankantsu]

Version 0.0.7 adds arbitrary Unicode character support in math expressions and now it is legal to write literal parentheses characters (( and )) in math expressions.
However, math-class for parentheses are not properly set now, and math expressions containing these characters cause incorrect spacing.

I know the use of math-paren is recommended way to write parentheses and when we use math-paren, spacing around parentheses is correct.
But, parentheses are used very often in math expressions, so the use of literal ( and ) is more handy.

Although We can use set-math-char primitive in our document file to set math-class for these characters to avoid the spacing problem, for often used symbols setting math-class in SATySFi itself seems reasonable.
For example, math-class of binary operators like + and relational operators like = are set in src/frontend/primitives.cppo.ml.

How about supporting literal parentheses characters officially by setting math-class correctly?

This PR sets math-class for parentheses symbols as follows.

• set ( (U+0028) to have math-class MathOpen
• set ) (U+0029) to have math-class MathClose

Here is a demo for spacing around parentheses.

@require: stdjareport
@require: list
@require: math

let correct-spacing ctx =
    % list of (cp-form, cp-to, math-class)
    let mkmap =
        [
            (0x0028, 0x0028, MathOpen);  % left paren
            (0x0029, 0x0029, MathClose); % right paren
        ]
    in
    mkmap |> List.fold-left
            (fun ctx (cp-from,cp-to,mk) -> (ctx |> set-math-char cp-from cp-to mk))
            ctx

let-inline ctx \correct-spacing m =
    embed-math (correct-spacing ctx) m

in

document (|
    title = {Awkward spacing around parentheses};
    author = {sankantsu};
|) '<
     +p {
         Default spacing: ${(+1) + (-2) = -1}
     }
     +p {
         Corrected spacing: \correct-spacing(${(+1) + (-2) = -1});
     }
     +p {
         Using `\paren`: ${\paren{+1} + \paren{-2} = -1}
     }
>

• before

• after

[gfngfn]

Thank you for suggestion! I will merge this PR.