Support for negative valued float constant

[Abhilekhgautam]

I was trying to lower to verilog from a calyx program that had negative float values.

I got hit with a parse error:

fud2 cnn.futil -o cnn.sv
FAILED: [code=1] ../cnn.sv 
/home/u/calyx/target/debug/calyx -l /home/u/calyx -b verilog  ../cnn.futil > ../cnn.sv
Error: ../cnn.futil
872 |    cst_2 = std_float_const(0, 32, -999999995904.000000);
       |                                                        ^ Parse error
expected bitwidth or float
ninja: build stopped: subcommand failed.
Error: ninja exited with exit status: 1

I think the grammar has a missing - sign:

// Floating-point numbers are only supported within the `std_float_const` primitive.
float =   @{ ASCII_DIGIT+ ~ ("." ~ ASCII_DIGIT+)? }