Underscores in Numeric Literals

Integers and floats should allow embedded underscores for readability.

The following are allowed:

1_000_000
1_000_
1____000
100.000_00
1.0__
1_0.01_0
0b1000_1011
0xCAFE_BABE
0b_1001_1010
0x_10_AB

The following are not:

_0x10
_0b10
0_x10
_10
1._9