Memory and speed optimizations

[xiam]

This PR contains different optimizations around memory usage.

This is the result of profiling the PostgreSQL test suite on master right now:

(pprof) top10
Showing nodes accounting for 2017.31MB, 68.05% of 2964.67MB total
Dropped 391 nodes (cum <= 14.82MB)
Showing top 10 nodes out of 151
      flat  flat%   sum%        cum   cum%
  516.63MB 17.43% 17.43%   547.13MB 18.46%  github.com/upper/db/v4/internal/sqlbuilder.expandQuery
  426.01MB 14.37% 31.80%  1085.03MB 36.60%  github.com/upper/db/v4/internal/cache/hashstructure.(*walker).visit
  206.50MB  6.97% 38.76%   206.50MB  6.97%  encoding/binary.Write
     195MB  6.58% 45.34%   392.51MB 13.24%  github.com/upper/db/v4/internal/cache/hashstructure.hashUpdateOrdered
  151.50MB  5.11% 50.45%   151.50MB  5.11%  reflect.(*structType).Field
  139.01MB  4.69% 55.14%   139.01MB  4.69%  reflect.packEface
  123.94MB  4.18% 59.32%   130.44MB  4.40%  github.com/upper/db/v4/internal/sqladapter.ReplaceWithDollarSign
   91.21MB  3.08% 62.39%    91.21MB  3.08%  github.com/jackc/chunkreader/v2.(*ChunkReader).newBuf (inline)
      85MB  2.87% 65.26%       85MB  2.87%  github.com/upper/db/v4/internal/sqladapter/exql.ColumnWithName
   82.51MB  2.78% 68.05%    94.03MB  3.17%  github.com/jackc/pgx/v4/stdlib.(*Rows).Next

Versus the result of profiling the same suite after applying optimizations:

(pprof) top10
Showing nodes accounting for 634.12MB, 45.92% of 1380.82MB total
Dropped 313 nodes (cum <= 6.90MB)
Showing top 10 nodes out of 176
      flat  flat%   sum%        cum   cum%
  120.44MB  8.72%  8.72%   120.44MB  8.72%  github.com/jackc/chunkreader/v2.(*ChunkReader).newBuf (inline)
   79.51MB  5.76% 14.48%    92.54MB  6.70%  github.com/jackc/pgx/v4/stdlib.(*Rows).Next
   67.29MB  4.87% 19.35%    99.79MB  7.23%  github.com/upper/db/v4/internal/sqlbuilder.expandQuery
   63.01MB  4.56% 23.92%    74.51MB  5.40%  encoding/json.Unmarshal
   62.08MB  4.50% 28.41%  1311.78MB 95.00%  github.com/upper/db/v4/adapter/postgresql.testPostgreSQLTypes
   58.11MB  4.21% 32.62%   185.11MB 13.41%  github.com/jackc/pgconn.(*PgConn).Prepare
   56.03MB  4.06% 36.68%    89.03MB  6.45%  github.com/upper/db/v4/internal/sqlbuilder.toColumnsValuesAndArguments
   44.63MB  3.23% 39.91%   255.11MB 18.48%  github.com/upper/db/v4/adapter/postgresql.(*database).CompileStatement
   42.53MB  3.08% 42.99%    48.03MB  3.48%  github.com/upper/db/v4/internal/sqlbuilder.Map
   40.51MB  2.93% 45.92%    40.51MB  2.93%  github.com/jackc/pgx/v4/stdlib.(*Rows).Columns```