Add the missing built-in constant-expression functions (#390)

[TheGupta2012]

Closes #390.

Problem

FUNCTION_MAP provided only 10 of the spec's built-in constant expression functions. ceiling, floor, exp, log, mod, popcount, rotl, and rotr all failed with the generic Invalid initialization value for constant 'c', in a const initializer and as a gate argument alike. The evaluator also passed only arguments[0], so no multi-argument function could work and sqrt(2.0, 3.0) was silently accepted.

Approach

• FUNCTION_MAP now maps each name to (implementation, arity). The evaluator dispatches through a single _evaluate_builtin_function, which checks arity, evaluates every argument, and turns a TypeError / ValueError from the implementation into a located error.
• rotl / rotr preserve the operand's declared width. The width comes from a BitValue, a bitstring literal, or the declaration of a bit[n] / uint[n] identifier. A widthless operand (rotl(37, 3)) is rejected rather than given a guessed width.
• A new FunctionCallError(ValidationError) covers an unknown name, a wrong arity, and a wrong argument type. raise_qasm3_error merges it into the statement-level message, so the user gets both the context and the function name: Invalid initialization value for constant 'c': Function 'sqrt' expects 1 argument(s), but 2 were given. One change site instead of one per wrapper, and no existing message is altered.
• _handle_function_init_expression emits a BitstringLiteral for a BitValue assigned to a bit[n], and an IntegerLiteral for an integral result. np.mod returns np.int64, which the old isinstance check missed, leaving mod(7, 2) un-unrolled in the output.

Tests

tests/qasm3/test_expressions.py, 22 new cases: every function in both a const initializer and a gate argument; rotl(a, n) == rotr(a, -n) and width preservation across rotations of 0, 1, 3, 8, 11, and -3; width recovery from a uint[8] declaration; arity errors for 1-, 2-, and 0-argument mistakes; type errors for ceiling("01"), rotl("1010", 1.5), popcount(1.5), and rotl(37, 3); and the unknown-name message.

pow

const int c = pow(2, 3); still fails at parse time in openqasm3 1.0.1, verified against the installed version — pow is also the gate-modifier keyword, so the grammar cannot tell the two apart.

No upstream issue should be filed. Upstream already removed pow from the spec for exactly this reason: openqasm/openqasm#627 proposed the deprecation, and openqasm/openqasm#635 landed it on 2025-10-15. ** is the supported spelling and pyqasm already handles it. A test pins both halves of that: pow(2, 3) raises a parse error, 2 ** 3 validates.

Stacked on #404 (feature-385-391-bit-repr-negative-indices); rebases onto main once #404 merges.

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