Align `substr` implementation with SQLite spec
(https://www.sqlite.org/lang_corefunc.html#substr):
> The substr(X,Y,Z) function returns a substring of input string X that
begins with the Y-th character and which is Z characters long. If Z is
omitted then substr(X,Y) returns all characters through the end of the
string X beginning with the Y-th. The left-most character of X is number
1. If Y is negative then the first character of the substring is found
by counting from the right rather than the left. If Z is negative then
the abs(Z) characters preceding the Y-th character are returned. If X is
a string then characters indices refer to actual UTF-8 characters. If X
is a BLOB then the indices refer to bytes.
Reviewed-by: Jussi Saurio (@jussisaurio)
Closes#1013
cbfd77849d