Out of range warning for as.integer64.character and hex support

[hcirellu]

as.integer64.character emits now out of range warnings and NA values for numerical character values not within integer64.
actual:

as.integer64("")
# integer64
# [1] 0

as.integer64("9223372036854775808")
# integer64
# [1] 9223372036854775807

this PR:

as.integer64("")
# integer64
# [1] <NA>

as.integer64("9223372036854775808")
# integer64
# [1] <NA>
# warning:
# In as.integer64.character("9223372036854775808") :
#   NAs introduced by coercion to integer64 range

In addition the support for hexadecimal values as in #173 is included as well.
actual:

as.integer64("0xF")
# integer64
# [1] <NA>

as.integer64("0x8000000000000000")
# integer64
# [1] <NA>

this PR:

as.integer64("0xF")
# integer64
# [1] 15

as.integer64("0x8000000000000000")
# integer64
# [1] <NA>
# warning:
# In as.integer64.character("0x8000000000000000") :
#   NAs introduced by coercion to integer64 range

Closes #17

[MichaelChirico]

@hcirellu as a side note, please push branches to this repo (r-lib/bit64), it makes things a bit easier

(e.g. the in-browser VSCode for some reason does not let me push to your fork)

[MichaelChirico]

LGTM, thanks!