Add #[inline] attribute to small functions

[AntoineSebert]

I tried to mark as inlined functions that I estimated suitable, up to 4 lines of codes or distinct language statements/expressions.
I discarded operator overloadings and new().
Hope that helps !

[phil-opp]

Looks good to me, thanks!

I just tried this PR on blog_os and it leads to a slightly smaller executable size:

Before:

  text	   data	    bss	    dec	    hex	filename
  47552	   2838	   8632	  59022	   e68e	target/x86_64-blog_os/release/blog_os

After:

   text	   data	    bss	    dec	    hex	filename
  46640	   2718	   8632	  57990	   e286	target/x86_64-blog_os/release/blog_os

[phil-opp]

Released as version 0.9.5

[m-ou-se]

Is there any reason the operator implementations (Add, Sub, etc.) should not be made #[inline] too, considering they are also mostly trivial functions? (If not, I'll send a PR for that.)

[phil-opp]

Given that the standard library does this, I think it would be fine if we do it too.