Widen int64 cache and compare integers without float conversion

[mattn]

Two changes for arithmetic-heavy scripts:

• The int64 value cache covered -1..256, so loop counters and sums beyond that boxed a new value on every operation. It now covers -1..4095 (~130KB resident).
• The <, <=, >, >= operators converted both sides to float64 even for integer operands. Integers are now compared as int64, which is faster and also fixes wrong results above 2^53 where float64 loses precision (e.g. 9223372036854775807 > 9223372036854775806 returned false).

BenchmarkLoop: 37.2us -> 35.3us, allocations 91 -> 23.