Support for joins

[fredrikroos]

Supports inner join, left/right/full outer join, cross join and implicit join. Join constraint can be ON expression, USING expression or NATURAL.

There is some formatting noise in this commit that can be dealt with by a cargo-fmt commit in master and then rebasing this PR.

[coveralls]

Pull Request Test Coverage Report for Build 78

• 376 of 421 (89.31%) changed or added relevant lines in 7 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.0%) to 82.217%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/sqlast/mod.rs	50	53	94.34%
tests/sqlparser_postgres.rs	88	94	93.62%
tests/sqlparser_generic.rs	122	132	92.42%
src/sqlparser.rs	110	136	80.88%

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_generic.rs	1	90.31%
src/sqlparser.rs	2	72.11%

Totals
Change from base Build 74:	1.0%
Covered Lines:	2492
Relevant Lines:	3031

---

💛 - Coveralls

[andygrove]

Thanks @fredrikroos I will review this over the weekend. It does look like there are some merge conflicts now. Sorry for the delay in reviewing.

[nickolay]

I rebased this on top of the current master, but can't share it in this PR, so here's a link to a commit in my fork: nickolay@9e9da81

This is roughly how I "rebased":

$ git reset --hard face972 # this PR's base
$ cargo fmt
$ # Some files were not reformatted in the PR, so undo the changes in them:
$ git checkout -- src/dialect/mod.rs
$ git checkout -- src/sqlast/value.rs
$ git commit -am .....
$ # Produce a patch without formatting changes:
$ git fetch upstream pull/28/head:join-support
$ git diff HEAD..join-support > join-support.patch
$ git checkout master
$ git apply join-support.patch
$ # Fixed conflict in the keywords list

[andygrove]

@fredrikroos I fixed the merge conflict using the web editor. There is now one failing test - could you take a look please?

[andygrove]

Thanks @fredrikroos I merged this manually and fixed the failing test