conversion to uint64 is bounded to int64 values (when passing as a literal)

[thehowl]

all of the following fail:

// 9223372036854775808 == 2**63 (first value not supported by int64)
var x = uint64(9223372036854775808)
var x uint64 = uint64(9223372036854775808)

const x = uint64(9223372036854775808)
const x uint64 = uint64(9223372036854775808)

$ gno run a.go
--- preprocess stack ---
stack 0: file{ package main; import fmt fmt; var x<VPBlock(2,1)> = (const-type uint64)((const (9223372036854775808 <untyped> bigint))); func main() { fmt<VPUverse(0)>.Pr
intln(x<VPUverse(0)>) } }
------------------------
panic: bigint overflows target kind [recovered]
        panic: main/a.go:5: bigint overflows target kind [recovered]
        panic: main/a.go:5: bigint overflows target kind

goroutine 1 [running]:
github.com/gnolang/gno/gnovm/pkg/gnolang.predefineNow.func1()
        /home/howl/oc/gno/gnovm/pkg/gnolang/preprocess.go:2851 +0x19c
panic({0xc89140, 0xc000316e80})
        /usr/lib/go/src/runtime/panic.go:884 +0x213
github.com/gnolang/gno/gnovm/pkg/gnolang.Preprocess.func2.1()
        /home/howl/oc/gno/gnovm/pkg/gnolang/preprocess.go:173 +0x46d
panic({0xbf0b00, 0xf4c640})
        /usr/lib/go/src/runtime/panic.go:890 +0x263
github.com/gnolang/gno/gnovm/pkg/gnolang.ConvertUntypedBigintTo(0xc0002ff770, {0xc000136018?}, {0xf58868, 0xf4b830})
        /home/howl/oc/gno/gnovm/pkg/gnolang/values_conversions.go:1070 +0x7a5
github.com/gnolang/gno/gnovm/pkg/gnolang.ConvertUntypedTo(0xc0002ff770, {0x0, 0x0})
        /home/howl/oc/gno/gnovm/pkg/gnolang/values_conversions.go:941 +0x8ff
github.com/gnolang/gno/gnovm/pkg/gnolang.convertConst({0xf651b0, 0xc000188240}, {0xf62cd8?, 0xc0002b38c0?}, 0xc0002ff740, {0x0?, 0x0?})
        /home/howl/oc/gno/gnovm/pkg/gnolang/preprocess.go:2355 +0x2b6
...

The only way to use a large literal like that is the following:

var x uint64 = 9223372036854775808
const x uint64 = 9223372036854775808

// values less than 2**63 are regularly supported
var x = uint64(9223372036854775807)

(Part of a series of issues encountered while developing gnochess)