tools: require function declarations

Except for arrow functions, require function declarations instead of
function expressions via linting. This is the predominant style in our
code base (77 instances of expressions to 2344 instances of
declarations).

.eslintrc.yaml

@@ -97,6 +97,7 @@ rules:
   eol-last: 2
   func-call-spacing: 2
   func-name-matching: 2
+  func-style: [2, declaration, {allowArrowFunctions: true}]
   indent: [2, 2, {ArrayExpression: first,
                   CallExpression: {arguments: first},
                   MemberExpression: 1,