bc: stop passing a pointer to G.prs down the call chain

bc: stop passing a pointer to G.prs down the call chain

function                                             old     new   delta
rewrite_label_to_current                              19      26      +7
bc_lex_assign                                         28      35      +7
zbc_lex_skip_if_at_NLINE                              14      19      +5
bc_parse_push                                         11      16      +5
bc_parse_operator                                    147     152      +5
bc_parse_create                                       92      97      +5
bc_lex_whitespace                                     38      43      +5
bc_lex_name                                           66      71      +5
bc_lex_lineComment                                    33      38      +5
zbc_lex_number                                       174     178      +4
zbc_parse_text_init                                   53      56      +3
zdc_parse_register                                    43      45      +2
zdc_parse_exprs_until_eof                             26      28      +2
bc_parse_free                                         38      40      +2
bc_lex_file                                           28      27      -1
zbc_parse_pushSTR                                     65      63      -2
bc_parse_expr_empty_ok                              1778    1776      -2
zbc_vm_execute_FILE                                   55      52      -3
bc_vm_init                                           678     675      -3
zbc_parse_stmt_allow_NLINE_before                     59      55      -4
bc_parse_pushNUM                                      80      74      -6
bc_parse_pushJUMP_ZERO                                27      21      -6
bc_parse_pushJUMP                                     27      21      -6
bc_vm_run                                            112     104      -8
bc_parse_pushName                                     39      31      -8
bc_parse_pushIndex                                    60      52      -8
zbc_parse_name                                       468     453     -15
zdc_program_execStr                                  524     501     -23
zdc_parse_mem                                         93      70     -23
zbc_program_exec                                    4003    3967     -36
zdc_parse_expr                                       518     476     -42
zbc_vm_process                                       923     878     -45
zbc_lex_next                                        2158    2070     -88
zbc_parse_stmt_possibly_auto                        1560    1451    -109
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 14/20 up/down: 62/-438)        Total: -376 bytes

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