parser, checker: fix generic fn variable assignment in generic fn (fix #7287)

[yuyi98]

This PR fix generic fn variable assignment in generic fn (fix #7287).

• Fix generic fn variable assignment in generic fn.
• Add test.

fn g[T]() u32 {
	return sizeof(T)
}

fn f[T]() u32 {
	p := g[T]
	return p()
}

fn main() {
	r1 := f[int]()
	println(r1)
	assert r1 == 4

	r2 := f[string]()
	println(r2)
	assert r2 == 16

	r3 := f[f64]()
	println(r3)
	assert r3 == 8

	r4 := f[bool]()
	println(r4)
	assert r4 == 1
}

PS D:\Test\v\tt1> v run .
4
16
8
1

fn g[T]() {
	println(sizeof(T))
}

fn f[T]() {
	p := &g[T]
	p()
}

fn main() {
	f[int]()
	f[string]()
}

PS D:\Test\v\tt1> v run .
4
16