use String::Builder for building query strings instead of Array.join

[repomaa]

fixes #181

[repomaa]

i'd be interested in a benchmark as well :)

[fridgerator]

Here are the benchmark results I get on this branch:

                           user     system      total        real
insert crecto :        0.530000   0.220000   0.750000 (  3.596781)
insert crystal-pg :    0.120000   0.120000   0.240000 (  2.957929)
                          user     system      total        real
update crecto :       0.680000   0.210000   0.890000 (  5.045013)
update crystal-pg :   0.170000   0.160000   0.340000 (  4.014328)
                           user     system      total        real
delete crecto :        0.390000   0.260000   0.650000 (  5.389259)
delete crystal-pg :    0.110000   0.150000   0.260000 (  3.905372)
                               user     system      total        real
select where crecto:       0.180000   0.140000   0.320000 (  1.931654)
select where crystal-pg:   0.090000   0.120000   0.210000 (  1.817702)

Compared to: https://github.com/Crecto/crecto/wiki/Benchmarks

So looks like a bit a performance increase.

[fridgerator]

FYI i'm just compiling this file with --release flag to benchmark