Add support for caller() in variable declaration

[GuillaumeGomez]

Fixes #506.

I think the other big missing feature. :)

Can you double-check @seijikun please?

Also cc @Kijewski since it's big feature (although in here, I mostly moved code around and copied write_let_block code with modifications).

[seijikun]

Awesome! This functionality is nice for macro call nesting, since this:

{%- macro outer() -%}
    {%- set caller_ = caller -%}
    <outer>
        {%- call inner() -%}
            {{- caller_() -}}
        {%- endcall -%}
    </outer>
{%- endmacro -%}

{%- macro inner() -%}
    <inner>
        {{- caller() -}}
    </inner>
{%- endmacro -%}


{%- call outer() -%}
    my content
{%- endcall -%}

can become this, which is a little easier to understand (IMHO):

{%- macro outer() -%}
    {%- set content = caller() -%}
    <outer>
        {%- call inner() -%}
            {{- content -}}
        {%- endcall -%}
    </outer>
{%- endmacro -%}

{%- macro inner() -%}
    <inner>
        {{- caller() -}}
    </inner>
{%- endmacro -%}


{%- call outer() -%}
    my content
{%- endcall -%}

Would it make sense to add a unit-test that checks that the output of these two variants is identical?
They differ, currently. I am not entirely sure why though.

[GuillaumeGomez]

Checking, might be a bug. But otherwise yes, gonna add a test to ensure both have the same output.

[GuillaumeGomez]

@seijikun That was a bug, but unrelated to this PR (I think?). Fixed it in the third commit.

[GuillaumeGomez]

Merging then. :)