Add support for `block` and `critical` constructs

[ZedThree]

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Fixes #320

Includes #325 as I used it in the tests

Restrictions C806, C810, C811 not implemented yet, which forbid certain other statements from appearing within the blocks. I wasn't sure how to do that -- if someone can point me at an example that does something similar, I can probably get those implemented too.

I've not added a test for nested block constructs introducing additional scopes, that's blocked by #326

[codecov[bot]]

Codecov Report

Base: 91.61% // Head: 91.64% // Increases project coverage by +0.03% :tada:

Coverage data is based on head (3dc7fc6) compared to base (8e2afd1). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #335      +/-   ##
==========================================
+ Coverage   91.61%   91.64%   +0.03%     
==========================================
  Files          37       37              
  Lines       13268    13324      +56     
==========================================
+ Hits        12155    12211      +56     
  Misses       1113     1113              

Impacted Files	Coverage Δ
src/fparser/two/parser.py	92.78% <ø> (ø)
src/fparser/two/Fortran2008.py	99.41% <100.00%> (+0.11%)	:arrow_up:

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[arporter]

Hi @ZedThree, would you mind bringing this up to master just to minimise the diffs that I need to review? Thanks :-)

[arporter]

This PR has been carried over to #335.