timeest: Add command for critical path estimation

[povik]

No description provided.

[povik]

Sample run:

$ yosys -p "read_verilog picorv32.v; prep -rdff; pmuxtree; techmap t:$shl; timeest -clk clk picorv32"

....

Domain \clk
Critical path is 99 nodes long:

    wire \latched_stalu source: picorv32.v:1198.6-1198.19 (level 0)
    cell $ternary$picorv32.v:1316$475 ($mux) source: picorv32.v:1316.23-1316.58
    wire $and$picorv32.v:1495$518_Y [1] source: picorv32.v:1495.36-1495.78 (level 4)
    cell $ternary$picorv32.v:1495$519 ($mux) source: picorv32.v:1495.20-1495.92
    wire $ternary$picorv32.v:1495$519_Y [1] source: picorv32.v:1495.20-1495.92 (level 8)
    cell $procmux$4994 ($mux) source: picorv32.v:0.0-0.0|picorv32.v:1493.5-1510.12
    wire $procmux$4994_Y [1] (level 12)
    cell $procmux$4997 ($mux) source: picorv32.v:0.0-0.0|picorv32.v:1481.3-1908.10
    wire $procmux$4997_Y [1] (level 16)
    cell $procmux$5000 ($mux) source: picorv32.v:1452.7-1452.14|picorv32.v:1452.3-1908.10
    wire $3\current_pc[31:0] [1] source: picorv32.v:1397.2-1970.5 (level 20)
    cell $add$picorv32.v:1564$561 ($add) source: picorv32.v:1564.22-1564.48
    wire $add$picorv32.v:1564$561_Y [31] source: picorv32.v:1564.22-1564.48 (level 83)
    cell $procmux$6062 ($mux) source: picorv32.v:1562.10-1562.19|picorv32.v:1562.6-1570.9
    wire $procmux$6062_Y [31] (level 87)
    cell $procmux$6064 ($mux) source: picorv32.v:1552.9-1552.24|picorv32.v:1552.5-1571.8
    wire $procmux$6064_Y [31] (level 91)
    cell $procmux$6075 ($mux) source: picorv32.v:0.0-0.0|picorv32.v:1481.3-1908.10
    wire $procmux$6075_Y [31] (level 95)
    cell $procmux$6078 ($mux) source: picorv32.v:1452.7-1452.14|picorv32.v:1452.3-1908.10
    wire $0\reg_next_pc[31:0] [31] source: picorv32.v:1397.2-1970.5 (level 99)