Consolidate multiple implementations of function call graphs across codebase

[TomAFrench]

We've gradually accumulated multiple implementations of a graph to track which functions are calling other functions during SSA.

Examples are:

noir/compiler/noirc_evaluator/src/ssa/opt/inlining.rs

Lines 309 to 344 in 0c46870

	fn compute_callers(ssa: &Ssa) -> BTreeMap<FunctionId, BTreeMap<FunctionId, usize>> {
	ssa.functions
	.iter()
	.flat_map(|(caller_id, function)| {
	let called_functions = called_functions_vec(function);
	called_functions.into_iter().map(|callee_id| (*caller_id, callee_id))
	})
	.fold(
	// Make sure an entry exists even for ones that don't get called.
	ssa.functions.keys().map(|id| (*id, BTreeMap::new())).collect(),
	|mut acc, (caller_id, callee_id)| {
	let callers = acc.entry(callee_id).or_default();
	*callers.entry(caller_id).or_default() += 1;
	acc
	},
	)
	}
	/// Compute for each function the set of functions called by it, and how many times it does so.
	fn compute_callees(ssa: &Ssa) -> BTreeMap<FunctionId, BTreeMap<FunctionId, usize>> {
	ssa.functions
	.iter()
	.flat_map(|(caller_id, function)| {
	let called_functions = called_functions_vec(function);
	called_functions.into_iter().map(|callee_id| (*caller_id, callee_id))
	})
	.fold(
	// Make sure an entry exists even for ones that don't call anything.
	ssa.functions.keys().map(|id| (*id, BTreeMap::new())).collect(),
	|mut acc, (caller_id, callee_id)| {
	let callees = acc.entry(caller_id).or_default();
	*callees.entry(callee_id).or_default() += 1;
	acc
	},
	)
	}

The purity callgraph added in #7197.

The callgraph to mark reachable globals in https://github.com/noir-lang/noir/blob/master/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_globals.rs added in #7188

We should see if we can standardize these to avoid code duplication and potential inconsistencies.