Properly raise SyntaxError for various mis-nested If/Elif/Else statements

[tpwrules]

This patch generalizes the fixes to #500 to correctly raise SyntaxError instead of silently doing the wrong thing for the statements Elif inside If, Else inside If, Elif inside Elif, and Else inside Else.

There are also new tests for these cases. Additionally there are new tests to verify the correct SyntaxError for Case inside Case, State outside of FSM, and State inside State (though their behavior was already correct and was not changed).

[codecov]

Codecov Report

Merging #610 (21147b0) into master (d824795) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #610   +/-   ##
=======================================
  Coverage   81.50%   81.50%           
=======================================
  Files          49       49           
  Lines        6461     6461           
  Branches     1287     1287           
=======================================
  Hits         5266     5266           
  Misses       1008     1008           
  Partials      187      187           

Impacted Files	Coverage Δ
nmigen/hdl/dsl.py	99.73% <100.00%> (ø)

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[whitequark]

Thanks!