ayazhafiz / hgb / 424
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ayazhafiz 
Distinguish statements and expressions

Gingerbread is an expression-based language whose control flow is guided
by statements. Currently, we parse all Gingerbread programs as a series
of expressions (which are evaluatable to a result), but in fact we
should parse programs as a series of statements (containers for
expression evaluations) to represent this control flow.

This commit distinguishes `Expression` and `Statement` grammars, and
moves the parser module to parse programs as a list of `Statement`s
rather than `Expression`s. In this commit, statements include expression
statements, variable declarations, and function declarations.

Because a sequence of statements in a block can serve as an expression
itself, this commit introduces a new expression: `BlockExpr`. A
`BlockExpr` consists of a series of statements followed by an
optional expression; the value of the `BlockExpr` is the value of
the final expression, or the unit type if there is no final expression.
`BlockExpr`s are useful as function or conditional bodies.
Following is an example listing of the values of a `BlockExpr`:

```gb
a : int  = if true { 1  } else { 2  } => a = 1
a : void = if true { 1; } else { 2; } => a is void
```

This commit also adds some TODOs to fix up inconsistencies in the
grammar introduced by the statement/expression differentiation. Most
notably, final expressions in `BlockExpr`s are currently not parsed.

Closes #40.

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ID Job ID Ran Files Coverage
1 424.1 0
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