Fix MustBeInt opcode semantics

[psvri]

In sqlite3 , we can create primary key with only integer column and during insert we can pass any value which can be parsed into integer as shown below. When I tried the same with limbo, it failed. The cause of this was due to MustBeInt opcode behaviour not aligned with sqlite. This PR aims to fix it.

Sqlite output

SQLite version 3.45.3
sqlite> create table temp (t1 integer, primary key (t1));
sqlite> insert into temp values (1),(2.0),('3'),('4.0');
sqlite> select * from temp;
1
2
3
4

Limbo output main branch

limbo>     create table temp (t1 integer, primary key (t1));
limbo>     insert into temp values (1),(2.0),('3'),('4.0');
Parse error: MustBeInt: the value in the register is not an integer
limbo>     select * from temp;
1

Limbo output with this PR

limbo> create table temp (t1 integer, primary key (t1));
limbo> insert into temp values (1),(2.0),('3'),('4.0');
limbo> select * from temp;
1
2
3
4