Added entries to explain expression-oriented languages

[Dangthrimble]

Added definitions for 'Expression', 'Expression-Oriented Language' and 'Statement' to glossary.
Sorted the definitions alphabetically.

r? @steveklabnik

[rust-highfive]

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[Dangthrimble]

Added the fact that expression statements can form part of larger expressions.

[Dangthrimble]

Tidied up glossary.md and added link from hello-world.md to 'expression-oriented language' in glossary.md

[tshepang]

missing comma between languages and expression

[tshepang]

Thanks for adding to the Glossary. It needs lots of love.

[steveklabnik]

Thanks so much, this is great :)

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 8fee567 has been approved by steveklabnik