pw_containers: Make Vector::at() use size_t

Unfortunately the size_type being different from size_t AND having the
size() and max() functions return size_t makes it difficult to NOT use a
size_t index when accessing the Vector. So, let's swing the other way
and take a size_t index when accessing the Vector.

Change-Id: Iea5d8600ae16294095f1c1085122485f0269066a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/215859
Reviewed-by: Wyatt Hepler <[email protected]>
Lint: Lint 🤖 <[email protected]>
Commit-Queue: Carlos Chinchilla <[email protected]>

pw_containers/public/pw_containers/vector.h

@@ -307,20 +307,20 @@ class Vector<T, vector_impl::kGeneric> {
 
   // Access
 
-  reference at(size_type index) {
+  reference at(size_t index) {
     PW_ASSERT(index < size());
     return data()[index];
   }
-  const_reference at(size_type index) const {
+  const_reference at(size_t index) const {
     PW_ASSERT(index < size());
     return data()[index];
   }
 
-  reference operator[](size_type index) {
+  reference operator[](size_t index) {
     PW_DASSERT(index < size());
     return data()[index];
   }
-  const_reference operator[](size_type index) const {
+  const_reference operator[](size_t index) const {
     PW_DASSERT(index < size());
     return data()[index];
   }