Add join() function

[zilder]

Rebased and a bit refactored version of #54. Couple of fixes include:

• fix allocation size for pairs array. It used to allocate more memory than it actually needed;
• remove redundant CreateTupleDescCopy() call;
• original function join() from previous PR was removed completely, and mjoin() renamed to join() as it is more universal.

Also this PR bumps version, so if we want to incorporate more changes to the SQL part of the extension, we can merge them together before merging to the master.

[codecov-io]

Codecov Report

Merging #82 into master will increase coverage by 0.06%. The diff coverage is 93.54%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   91.59%   91.65%   +0.06%     
==========================================
  Files          10       10              
  Lines        1940     2002      +62     
==========================================
+ Hits         1777     1835      +58     
- Misses        163      167       +4

Impacted Files	Coverage Δ
src/bigistore.c	95.44% <93.54%> (-0.21%)	:arrow_down:

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[alexeyklyukin]

Please, merge in changes from the master. Can't compile on 12dev ( error: unknown type name 'FunctionCallInfoData').