bc: move BcLex::lex member to be the first in struct globals

bc: move BcLex::lex member to be the first in struct globals

function                                             old     new   delta
zbc_program_exec                                    3967    4003     +36
zdc_program_execStr                                  501     520     +19
zbc_posix_error_fmt                                   58      62      +4
bc_vm_init                                           675     679      +4
bc_read_line                                         407     411      +4
bc_error_fmt                                          36      40      +4
zdc_parse_register                                    45      44      -1
zdc_parse_exprs_until_eof                             28      27      -1
zbc_parse_text_init                                   56      55      -1
zbc_parse_stmt_allow_NLINE_before                     55      54      -1
zbc_lex_skip_if_at_NLINE                              19      18      -1
zbc_lex_number                                       178     177      -1
bc_parse_create                                       97      96      -1
bc_lex_whitespace                                     43      42      -1
bc_lex_name                                           71      70      -1
bc_lex_lineComment                                    38      37      -1
bc_lex_assign                                         35      34      -1
zdc_parse_expr                                       476     473      -3
bc_verror_msg                                         93      90      -3
bc_lex_file                                           27      24      -3
zbc_parse_name                                       453     448      -5
bc_parse_expr_empty_ok                              1776    1764     -12
zbc_vm_process                                       878     865     -13
zbc_parse_stmt_possibly_auto                        1451    1425     -26
zbc_lex_next                                        2075    2039     -36
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 6/19 up/down: 71/-112)          Total: -41 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>