Improve grammar of block expressions

[traviscross]

At some point, we may want to improve the grammar for blocks.

The way that rustc works is that a block is a sequence of statements. It's probably clearer to do something like that here. Then we can just say that "all expressions are statements" and that "a block is a sequence of statements that are delimited by semicolons with an optional trailing semicolon" or something to that effect.

But, of course, we should beware of:

• What is a trailing expression in a block exactly? rust#61733
• Document that expressions in non-trailing expression statements without semicolon must have type () #625

cc @ehuss