Add flooring division (//) and modulus (%) operators

[AaronWebster]

Part 1 of 3 of the integer division/modulus series (base: master), following
doc/design_docs/division_and_modulus.md.

Adds integer // (flooring division) and % (flooring modulus) to the Emboss
expression language.

What this does

• Front end: ///% tokens; the grammar splits the multiplicative tier
  into a *-chain and a ///%-chain that may not be mixed without
  parentheses (per the design doc); type checking; and expression-bounds
  computation.
• Semantics: flooring, matching Python 3 (-7 // 2 == -4, -7 % 2 == 1).
  Constant folding reuses Python's ///%; the runtime FlooringQuotient /
  FlooringRemainder helpers convert C++'s truncating / and % for signed
  types (unsigned needs no conversion).
• Division by zero: this initial rollout does not yet track an undefined
  value through the type system, so a divisor whose range includes zero is
  rejected at compile time. constant_value() folds ///% totally (a zero
  divisor yields no value instead of raising), so the diagnostic is reported
  cleanly rather than as a traceback.
• Docs, unit tests (tokenizer, grammar, bounds, constraints, constant-folding,
  runtime arithmetic), an integration schema + golden, and formatter coverage.

Series

1. This PR — the operators, with strict division-by-zero rejection.
2. Relax to allow possibly-zero divisors, propagating an undefined value.
3. IsComplete() returns true for a division-by-zero size.

Supersedes the stale WIP #263.

bazel test //... is green.

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PR chain (merge in order): #273 (this) → #274 → #275