fixed_point: Make to_uint() non-const

This calls round_up(), which is a non-const member function, so if a
fixed-point instantiation ever calls to_uint(), it'll result in a
compiler error.

This allows the member function to work.

While we're at it, we can actually mark to_long_floor() as const, since
it's not modifying any member state.

src/common/fixed_point.h

@@ -411,7 +411,7 @@ public: // conversion to basic types
         return static_cast<int>((data_ & integer_mask) >> fractional_bits);
     }
 
-    constexpr unsigned int to_uint() const {
+    constexpr unsigned int to_uint() {
         round_up();
         return static_cast<unsigned int>((data_ & integer_mask) >> fractional_bits);
     }
@@ -425,7 +425,7 @@ public: // conversion to basic types
         return static_cast<int>((data_ & integer_mask) >> fractional_bits);
     }
 
-    constexpr int64_t to_long_floor() {
+    constexpr int64_t to_long_floor() const {
         return static_cast<int64_t>((data_ & integer_mask) >> fractional_bits);
     }