Dynahash: init support for Postgres Hash tables. (#22)

Add typed wrappers for postgres HTAB type (`dynahash` hash tables that
can also be used with shared memory).

When using HTAB one confiures an 'Entry' that can be any kind of struct.
The prefix (first field) of the struct is supposed to hold the key. For
string keys this means that we have to configure a max string length.

Author: urso

src/pgzx.zig

@@ -5,16 +5,32 @@
 
 const std = @import("std");
 
-// pub const Build = @import("pgzx/build.zig");
-
 // Export common set of postgres headers.
 pub const c = @import("pgzx/c.zig");
 
 // Utility functions for working with the PostgreSQL C API.
 pub const bgworker = @import("pgzx/bgworker.zig");
+
+pub const collections = @import("pgzx/collections.zig");
+pub const PointerListOf = collections.list.PointerListOf;
+pub const SList = collections.slist.SList;
+pub const DList = collections.dlist.DList;
+pub const HTab = collections.htab.HTab;
+pub const HTabIter = collections.htab.HTabIter;
+pub const StringHashTable = collections.htab.StringHashTable;
+pub const KVHashTable = collections.htab.KVHashTable;
+
 pub const elog = @import("pgzx/elog.zig");
+
 pub const err = @import("pgzx/err.zig");
+pub const PGError = err.PGError;
+pub const pgRethrow = err.pgRethrow;
+
 pub const fmgr = @import("pgzx/fmgr.zig");
+pub const PG_MODULE_MAGIC = fmgr.PG_MODULE_MAGIC;
+pub const PG_FUNCTION_V1 = fmgr.PG_FUNCTION_V1;
+pub const PG_FUNCTION_INFO_V1 = fmgr.PG_FUNCTION_INFO_V1;
+
 pub const lwlock = @import("pgzx/lwlock.zig");
 pub const mem = @import("pgzx/mem.zig");
 pub const pq = @import("pgzx/pq.zig");
@@ -25,17 +41,4 @@ pub const utils = @import("pgzx/utils.zig");
 pub const intr = @import("pgzx/interrupts.zig");
 pub const testing = @import("pgzx/testing.zig");
 
-// data structures
-pub const collections = @import("pgzx/collections.zig");
-
 pub const guc = utils.guc;
-
-pub const PGError = err.PGError;
-pub const pgRethrow = err.pgRethrow;
-
-pub const PG_MODULE_MAGIC = fmgr.PG_MODULE_MAGIC;
-pub const PG_FUNCTION_V1 = fmgr.PG_FUNCTION_V1;
-pub const PG_FUNCTION_INFO_V1 = fmgr.PG_FUNCTION_INFO_V1;
-
-pub const list = collections.list;
-pub const PointerListOf = list.PointerListOf;

src/pgzx/collections.zig

@@ -1,3 +1,4 @@
 pub const list = @import("collections/list.zig");
 pub const slist = @import("collections/slist.zig");
 pub const dlist = @import("collections/dlist.zig");
+pub const htab = @import("collections/htab.zig");