Meta: proper elementary math support

Properly implement elementary math support (spec)

mstack
- align rows of digits and operators
msgroup
- groups rows with similar alignment
msrow
- groups digits and operators into a row
msline
- draws lines between rows of the stack
mscarries
- annotates the following row with optional borrows/carries and/or crossouts
mscarry
- a borrow/carry and/or crossout for a single digit
mlongdiv
- specifies a divisor and a quotient for long division, along with a stack of the intermediate computations
some TeX macros to match