AIR:
 * div is renamed to div_trunc.
 * Add div_float, div_floor, div_exact.
   - Implemented in Sema and LLVM codegen. C backend has a stub.

Improvements to std.math.big.Int:
 * Add `eqZero` function to `Mutable`.
 * Fix incorrect results for `divFloor`.

Compiler-rt:
 * Add muloti4 to the stage2 section.