Updated primitive data types: added `u128`

[elementh]

Closes #18

[dumindu]

@elementh Thanks for informing about this missing type. I think PR will be more complete if you can add i128 as well.

Files we need to update,

• https://github.com/learning-rust/site/blob/master/source/docs/a8.primitive_data_types.md : title & tables which shows min/ max values.
• https://github.com/learning-rust/site/blob/master/source/docs/d7.std_primitives_and_preludes.md : need to remove comments // i128 : nightly-only experimental API and // u128 : nightly-only experimental API; two places each.

[elementh]

@dumindu done! I also had to update the max-width since the site was broken with the width of the new row at the table.

Hope it's alright! Tell me if I need to correct something :)

[dumindu]

@elementh Instead changing max-width, I think it's better to add overflow-x: scroll to .article-content themes/navy/source/css/_partial/page.styl : line 123 :)

Also please update source/docs/a8.primitive_data_types.md : line 41 and 27

[elementh]

@dumindu Are you sure about the overflox-x: scroll? I did try that before max-width but it feels so wrong, and 100px don't really break anything; on the other side, that scroll-x feels like a rushed fix, at least in my screen.

[dumindu]

@elementh sorry for the very late response buddy. Let me check!

[elementh]

@dumindu no problem! Glad to help :)

[dumindu]

@elementh I didn't think that you reply to this quickly, and also I didn't want to make you wait to add another file change after all most 10 days because of my fault. Sorry for updating styles by myself.

Actually we need overflow-x: scroll to fix styles on tablets. Also I prefer to keep max-width:1200px because it affects to width of the paragraphs :)