Move block statement to statement

[leewei05]

• Move block statement to statement
• Support single statement for if statement
• Fix dangling else problem

[Lai-YT]

I believe it's essential to distinguish between blocks and normal statements since blocks cannot always be replaced by a statement. Consider the main_func as an example, which must have its body enclosed within a pair of curly braces.
Allowing any statement to represent its body could result in syntactically valid, yet unintended code like the following Python-ish syntax:

int main()
    return 0;

For reference, in the C standard ISO/IEC 9899:1999, section 6.9.1 defines the syntax of a function-definition:

• function-definition:
  declaration-specifiers declarator declaration-list_opt compound-statement

Here, the compound-statement corresponds to our block-statement.

[Lai-YT]

Let's create a test for the if_else_single_stmt, possibly a nested one as well!

[Lai-YT]

Oh yeah, for the proper support of the if statement! 🎉
We're now closer to the moon. 🚀