Slow performance on bulk insert

Bulk inserts appear to take quadratic time over the number of statements being processed. Using v1.14.16, I find that a bulk insert that finishes quickly for the sqlite3 shell does not finish at all when run in golang.

My supposition is that this is because the sqlite3_prepare_v2 function returns a pointer to the remaining unprocessed sql, which we then copy back into a Go string. On the next run of the loop in exec(), we do another roundtrip of copying strings Go->C->Go. By contrast, the sqlite3 shell performs the same task without copying the underlying buffer, and finishes significantly faster.

A representative stack trace shows indicates time spent in runtime.memmove():

runtime.memmove()
	src/runtime/memmove_amd64.s:432 +0x513 fp=0xc0005db3d0 sp=0xc0005db3c8 pc=0x19fca13
github.com/mattn/go-sqlite3._Cfunc_CString({0xc00a3d6001, 0x9254ac8})
	_cgo_gotypes.go:200 +0x7b fp=0xc0005db400 sp=0xc0005db3d0 pc=0x27ee11b
github.com/mattn/go-sqlite3.(*SQLiteConn).prepare(0xc0010ca000, {0xb2ff30?, 0xc00021a008?}, {0xc00a3d6001?, 0x0?})
	rh/external/com_github_mattn_go_sqlite3/sqlite3.go:1805 +0x5e fp=0xc0005db490 sp=0xc0005db400 pc=0x27fff3e
github.com/mattn/go-sqlite3.(*SQLiteConn).exec(0x0?, {0xb2ff30, 0xc00021a008}, {0xc0177da000?, 0x0?}, {0x3433fd0, 0x0, 0x0})
	rh/external/com_github_mattn_go_sqlite3/sqlite3.go:853 +0x99 fp=0xc0005db5d8 sp=0xc0005db490 pc=0x27f9939
github.com/mattn/go-sqlite3.(*SQLiteConn).ExecContext(0xc0005db6c8?, {0xb2ff30, 0xc00021a008}, {0xc0177da000, 0x9255f37}, {0x3433fd0, 0x0, 0xc0010ca000?})
	sqlite3_go18.go:41 +0xbb fp=0xc0005db670 sp=0xc0005db5d8 pc=0x27edb5b

Start by reproducing the bulk-insert slowdown described in the issue, then inspect sqlite3.go around prepare (line 1805) and exec (line 853), along with sqlite3_go18.go. Trace the Go-to-C string handling highlighted by the stack trace and compare the runtime with the sqlite3 shell. Done means the representative bulk insert no longer shows quadratic behavior.