exercises: stubs: resolve unused parameter errors

[ee7]

Before this commit, when testing an unchanged exercise stub:

$ zig test test_leap.zig
leap.zig:1:19: error: unused function parameter
pub fn isLeapYear(year: u32) bool {
                  ^~~~

With this commit:

$ zig test test_leap.zig
Test [1/9] test.year not divisible by 4 in common year... thread 123456 panic: please implement the isLeapYear function
/foo/exercism-tracks/zig/exercises/practice/leap/leap.zig:3:5: 0x201234 in isLeapYear (test)
    @panic("please implement the isLeapYear function");
[...]

Closes: #175

---

Refs: #143

With this PR, there are no more unused parameter errors when running zig fmt . --ast-check. This gets us closer to checking stubs in CI.

$ zig fmt . --ast-check
./exercises/practice/binary-search/binary_search.zig:3:60: error: use of undeclared identifier 'SearchError'
pub fn binarySearch(target: usize, buffer: ?[]const usize) SearchError!usize {
                                                           ^~~~~~~~~~~
./exercises/practice/grains/grains.zig:1:29: error: use of undeclared identifier 'ChessboardError'
pub fn square(index: isize) ChessboardError!u64 {
                            ^~~~~~~~~~~~~~~
./exercises/practice/hamming/hamming.zig:1:55: error: use of undeclared identifier 'DnaError'
pub fn compute(first: []const u8, second: []const u8) DnaError!usize {
                                                      ^~~~~~~~
./exercises/practice/proverb/proverb.zig:1:26: error: use of undeclared identifier 'mem'
pub fn recite(allocator: mem.Allocator, words: []const []const u8) (fmt.AllocPrintError || mem.Allocator.Error)![][]u8 {
                         ^~~
./exercises/practice/resistor-color-duo/resistor_color_duo.zig:1:34: error: use of undeclared identifier 'ColorBand'
pub fn colorCode(colors: []const ColorBand) anyerror!isize {
                                 ^~~~~~~~~
./exercises/practice/resistor-color/resistor_color.zig:1:25: error: use of undeclared identifier 'ColorBand'
pub fn colorCode(color: ColorBand) isize {
                        ^~~~~~~~~
./exercises/practice/resistor-color/resistor_color.zig:5:25: error: use of undeclared identifier 'ColorBand'
pub fn colors() []const ColorBand {
                        ^~~~~~~~~
./exercises/practice/rna-transcription/rna_transcription.zig:3:25: error: use of undeclared identifier 'mem'
pub fn toRna(allocator: mem.Allocator, dna: []const u8) (RnaError || mem.Allocator.Error)![]const u8 {
                        ^~~
./exercises/practice/secret-handshake/secret_handshake.zig:1:38: error: use of undeclared identifier 'mem'
pub fn calculateHandshake(allocator: mem.Allocator, number: isize) mem.Allocator.Error![]const Signal {
                                     ^~~
./exercises/practice/space-age/space_age.zig:12:58: error: use of undeclared identifier 'Planet'
    fn getOrbitalPeriodInSecondsFromEarthYearsOf(planet: Planet) f64 {
                                                         ^~~~~~
./exercises/practice/space-age/space_age.zig:17:47: error: use of undeclared identifier 'Planet'
    fn getOrbitalPeriodInEarthYearsOf(planet: Planet) f64 {
                                              ^~~~~~
./exercises/practice/triangle/triangle.zig:4:54: error: use of undeclared identifier 'TriangleError'
    pub fn init(first: f64, second: f64, third: f64) TriangleError!Triangle {
                                                     ^~~~~~~~~~~~~
./exercises/practice/triangle/triangle.zig:11:79: error: use of undeclared identifier 'TriangleError'
    fn verifyIfDegenerateAttributesExist(first: f64, second: f64, third: f64) TriangleError!void {
                                                                              ^~~~~~~~~~~~~
./exercises/practice/triangle/triangle.zig:18:77: error: use of undeclared identifier 'TriangleError'
    fn verifyIfTriangleInequalityHolds(first: f64, second: f64, third: f64) TriangleError!void {
                                                                            ^~~~~~~~~~~~~