Evaluate pi_ and other constants as floats

[fllor]

This PR addresses some issues discussed during the workshop.

• Add Euler and Euler-Mascheroni constants as builtin symbols (ee_, em_)
• Allow Evaluate statement to insert arbitrary precision floating point values for the three builtin constants (π, e, γ). The values are provided (and cached) by MPFR
• Allow symbols as arguments to Evaluate statement, e.g. Evaluate pi_
• Test cases are included for various expressions, such as pi_*sqrt_(3), sqrt_(pi_), pi_*ee_

Furthermore, this PR fixes a bug in the printing of floating point numbers, that could cause exponents to be truncated and memory errors that would be lead to warnings in valgrind.

[coveralls]

coverage: 50.581% (+0.6%) from 49.999%
when pulling 6880a65 on fllor:master
into 83e3d41 on vermaseren:master.

[fllor]

It seems the failure of the 32-bit tests might be due to a bug somewhere in the floating point engine. The program

#StartFloat 64
    
Local X = 2/3;
Local Y = sqrt_(3);
Local Z = pi_;
    
ToFloat;
Evaluate;
    
Print;
.end

gives me

X = 6.66666666666666666667e-01;
Y = 1.73205080756887729352e+00;
Z = 3.14159265358979323846e+00;

on my laptop and

X = 6.66666666666666666666666666667e-;
Y = 1.7320508075688772935274463415e+0;
Z = 3.14159265358979323846264338328e+;

on the 32-bit containers.

For some reason, floats are evaluated to a different precision.
Also, note that Y in the first case is not rounded correctly. In the second case the exponents are not printed properly.

I am not sure if this is a bug in Form or GMP/MPFR. The internal representations are already different in evaluate.c:668.

[jodavies]

There is a known problem with the truncation of trailing zeros and messing up the exponent (see #492) which looks to be also the cause of the valgrind failures in your tests. So that needs to be sorted out but I didn't re-check it yet.

Jos said in his slides that float mode is not expected to work properly in 32bit mode, and it seems that it will be removed/declared "unsupported" for v5 anyway.

Once the valgrind errors are resolved you could also move your tests to features.frm rather than user-tests.frm.

[fllor]

Printing of exponents and valgrind errors should be fixed now. 32-bit tests will probably still fail.