Allow to mix statements and declarations in a single line

[MartinNowak]

Proposed by @callumenator, https://github.com/MartinNowak/drepl/pull/14#commitcomment-5392071.

[MartinNowak]

If the declarations were parsed as DeclarationStatement, this should already work, but it doesn't. string foo(){ return "bar"; } writeln(foo());