Remove garbage
package windows: upgrade to LZ4 r125

The release has a fix for cross-building for Windows on GNU/Linux.
Replace zlib to LZ4

They are copy & paste misses.
Use pkg-config to detect LZ4
cmake: use pkg-config to detect LZ4

It maybe works...
cmake: need to use LZ4 information extracted by pkg-config

The previous change didn't work...
doc: fix wrong section
Fix comment

It seems that GRN_UVECTOR isn't an array of grn_id. It may be an array
fixed size data such as GRN_DB_UINT64.
Add grn_uvector_element_size()
grn_uvector_size() supports non grn_id fixed size type such as GRN_DB_UINT64
Support indexing vector of fixed size data such as vector of UInt64

[groonga-dev,03014]

Reported by funa 1g. Thanks!!!
test: add a test for indexing vector of UInt64
test: group tests by scalar or vector
test: add a test for indexing vector of geo point
package debian: add missing "[ ... ]"
Initialize ctx->impl in grn_ctx_init()

Remove lazy ctx->impl initialization feature because it's not useful for
now.
Remove a needless empty line
mrb: bind grn_db
mrb: bind grn_obj_close()
mrb: bind grn_db_open() and grn_db_create()