Improve compiler warnings with #[must_use]

Hello,

After porting my project to nalgebra, I spent way too much time debugging a black screen, which I eventually traced back to my following mistake:
```rust
view_matrix.prepend_translation(&(-camera));
```
This function returns a new matrix; the line is a no-op.

Enter [#[must_use]](https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute):
```rust
#[inline]
#[must_use = "Did you mean to use prepend_translation_mut()?"]
pub fn prepend_translation(
...
```
This turns a silent error into a helpful compiler warning:
```rust
| view_matrix.prepend_translation(&(-camera));
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: #[warn(unused_must_use)] on by default
= note: Did you mean to use prepend_translation_mut()?
```

I propose to tag all functions that return a new object without side effects as #[must_use], optionally with a hint towards an in-place alternative.

This might also help with #534.

Is this something you'd consider an improvement? Shall I try to prepare a pull request, or would you rather go through your code yourself?