Add stubs to some exercises (#339)

Author: leoyvens

exercises/beer-song/src/lib.rs

@@ -0,0 +1,7 @@
+pub fn verse(n: i32) -> String {
+
+}
+
+pub fn sing(start: i32, end: i32) -> String {
+
+}

exercises/bob/src/lib.rs

@@ -0,0 +1,3 @@
+pub fn reply(message: &str) -> &str {
+
+}

exercises/difference-of-squares/src/lib.rs

@@ -0,0 +1,11 @@
+pub fn square_of_sum(n: usize) -> usize {
+
+}
+
+pub fn sum_of_squares(n: usize) -> usize {
+
+}
+
+pub fn difference(n: usize) -> usize {
+
+}

exercises/leap/src/lib.rs

@@ -0,0 +1,3 @@
+pub fn is_leap_year(year: i32) -> bool {
+
+}

exercises/proverb/src/lib.rs

@@ -0,0 +1,3 @@
+pub fn build_proverb(list: Vec<&str>) -> String {
+
+}

exercises/raindrops/src/lib.rs

@@ -0,0 +1,3 @@
+pub fn raindrops(n: usize) -> String {
+
+}

exercises/sum-of-multiples/src/lib.rs

@@ -0,0 +1,3 @@
+pub fn sum_of_multiples(limit: u32, factors: &[u32]) -> u32 {
+
+}