add string functions, cleanup vector docs

docs/string.md

@@ -0,0 +1,40 @@
+# string
+
+C string helpers
+
+## Functions
+
+### find_substrings
+Finds the indexes of all locations of the given needle in the haystack. The returned array of `size_t` is allocated by
+this function and memory cleanup is managed by the user. `num_substrings` is a pointer to a pre-allocated `size_t` that will
+be modified to contain the number of found substrings and subsequently the size of the returned array of `size_t`.
+Returns `NULL` and prints to `stderr` if there is an error.
+
+```c
+size_t *find_substrings(const char* haystack, const char* needle, size_t *num_substrings);
+
+/* Usage */
+const char* haystack = "One two three two";
+const char* needle = "two";
+size_t subs_sz = 0;
+
+size_t *subs = find_substrings(haystack, needle, &subs_sz);
+// subs: [ 4, 14 ]
+// subs_sz: 2
+
+free(subs);
+```
+
+### substr
+
+Extracts a substring at a specific index and length. This function returns a copy of the substring in a heap allocated
+buffer that the user is responsible for freeing. Returns `NULL` and prints to `stderr` if there is an error.
+
+```c
+const char* substr(const char* str, size_t idx, size_t len);
+
+/* Usage */
+const char *s = substr("One two three", 4, 3);
+assert(strcmp(s, "two") == 0);
+free(s);
+```

docs/vector.md

@@ -117,7 +117,7 @@ Shrinks the capacity of the vector down to the current length. Returns a non-zer
 int vec_shrink(Vector *vec);
 ```
 
-## vec_max
+### vec_max
 
 Finds the largest value in the vector and returns a void pointer to the underlying data. Requires a 
 comparison function to compare the data in the vector. This function must return `1` if `a > b`, `-1` if 
@@ -127,7 +127,7 @@ comparison function to compare the data in the vector. This function must return
 const void *vec_min(const Vector *vec, int(*cmp)(const void *a, const void *b));
 ```
 
-## vec_min
+### vec_min
 
 Finds the smallest value in the vector and returns a void pointer to the underlying data. Requires a 
 comparison function to compare the data in the vector. This function must return `1` if `a > b`, `-1` if