Support parsing int64 in Blueprint file.

Support int64 number instead of int to be more fixed to bit size so
that the underlying arch won't affect overflow cases. Besides,
refection: func (v Value) Int() int64 always cast to int64 no matter the
input is int, int16, int32. Currently we always treat "-" as negative
sign to bind to next value, and "+" as plus operator to add operands
together.
So we allow:
a = 5 + -4 + 5 or a = -4 + 5
But we don't allow:
a = +5 + 4 + -4 since we don't treat "+" as a positive sign, otherwise,
a = 5 + +5 would exist which looks pretty weird. In the future, we may
want fully support number calculator logic eg, "+"/"-" can be
positive/negative sign or operator, and "(" and ")" will be considered
to group expressions with a higher precedence.

int & uint properties within struct keeps unchanged, which is only
allowed when tagged with 'blueprint:mutated'. We only allow *int64
property instead of int64 property within struct since it does't make
sense to do prepending or appending to int64.

Change-Id: I565e046dbd268af3538aee148cd7300037e56523