Validate arity of labels in br_table

[gumb0]

Required for #354.

There's a slight complication around loop's arity: turns out if there's loop in br_table labels, it's arity is considered always 0 (and then br_table is valid only if all labels have arity 0).
It's difficult to find clear confirmation of this in the spec, but spec tests cover it in this case https://github.com/WebAssembly/spec/blob/636b862b9c8a25ad65fb240fefd673e7f23bcdd0/test/core/br_table.wast#L1229-L1244

[codecov]

Codecov Report

Merging #368 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #368      +/-   ##
==========================================
+ Coverage   98.85%   98.86%   +0.01%     
==========================================
  Files          42       42              
  Lines       12184    12320     +136     
==========================================
+ Hits        12044    12180     +136     
  Misses        140      140              

[gumb0]

I will squash test commits before merging.

[chfast]

Suggested change

	throw validation_error{"br_table labels have inconsistent block result types"};
	throw validation_error{"br_table labels have inconsistent types"};

... as wabt is reporting this...

[gumb0]

Changed error message and squashed test commits.

[gumb0]

Rebased.