Fix mathML(Type) -- previously caused a recursion error

[d-torrance]

Currently, we have:

i1 : mathML Matrix
stdio:1:6:(3): error: recursion limit of 300 exceeded

What's going on?

i2 : code (mathML, Type)

o2 = -- code for method: mathML(Type)
     /usr/share/Macaulay2/Core/mathml.m2:167:24-167:80: --source code:
     mathML ImmutableType := R -> if R.?mathML then R.mathML else mathML expression R

i3 : peek expression Matrix

o3 = Holder{Matrix}

i4 : code (mathML, Holder)

o4 = -- code for method: mathML(Holder)
     /usr/share/Macaulay2/Core/mathml.m2:40:17-40:32: --source code:
     mathML Holder := v -> mathML v#0

Since there's no Matrix.mathML, we just bounce back and forth between mathML(Type) and mathML(Holder).

We fix this by just removing mathML(Type) and using inheritance so it calls mathML(MutableHashTable) (and also setting this to be mathML(ImmutableType)), which does the expected thing:

i1 : mathML Matrix

o1 = <mi>Matrix</mi>

[d-torrance]

This (kind of) broke mathML for polynomial rings. This is the current behavior:

i1 : mathML(QQ[a,c,e])

o1 = <mi>ℚ</mi> <mrow><mo>[</mo><mrow><mi>a</mi><mo>,</mo><mi>c</mi><mo>,</mo><
     mi>e</mi></mrow><mo>]</mo></mrow>

Which renders as:

ℚ [a,c,e]

(Yay GitHub can render mathml in comments!)

But notice that I used variable names that aren't adjacent alphabetically. When we do that:

i2 : mathML(QQ[a,b,c])
stdio:2:6:(3): error: mathML conversion for expression class BinaryOperation not implemented yet

That's because expression inserts a .. operator between a and c, and mathML chokes on that.

With just the first commit from this PR, we get the following silly thing:

i1 : mathML(QQ[a,b,c])

o1 = <mrow><mi>PolynomialRing</mi><mtext>{...55...}</mtext></mrow>

So we add mathML(EngineRing) and mathML(BinaryOperation), giving us the expected result:

i1 : mathML(QQ[a,b,c])

o1 = <mi>ℚ</mi> <mrow><mo>[</mo><mrow><mrow><mi>a</mi><mo>..</mo><mi>c</mi></
     mrow></mrow><mo>]</mo></mrow>

This renders as:

ℚ [a..c]

[d-torrance]

I ended up just dropping mathML(ImmutableType) completely. It was redundant since Module is the only child of ImmutableType and mathML(Module) is defined elsewhere. None of the other text formatting methods are installed for ImmutableType either.

[d-torrance]

Discussed with Mike