gr_nf and gr_fmpz_poly crash in WASM

[oscarbenjamin]

In gh-318 the doctests for gr_nf and gr_fmpz_poly have to be commented out because the otherwise crash with:

RuntimeError: memory access out of bounds
    at wasm://wasm/020eeec2:wasm-function[9770]:0x4883e2
    at wasm://wasm/02dce51a:wasm-function[15598]:0x92739c
    at wasm://wasm/02dce51a:wasm-function[15582]:0x926e29
    at wasm://wasm/02dce51a:wasm-function[14735]:0x8d1694
    at wasm://wasm/02fbdcda:wasm-function[421]:0xc4097
    at wasm://wasm/020eeec2:wasm-function[2306]:0x1b8b1d
    at wasm://wasm/02fbdcda:wasm-function[684]:0xf209f
    at wasm://wasm/020eeec2:wasm-function[2306]:0x1b8b1d
    at wasm://wasm/020eeec2:wasm-function[1962]:0x1a0d0a
    at wasm://wasm/020eeec2:wasm-function[1949]:0x1a01c4 {
  pyodide_fatal_error: true
}

This was the only issue that did not get resolved in gh-262.